package nom.tam.fits.header;
   
   import nom.tam.fits.AsciiTable;
   import nom.tam.fits.BinaryTable;
   import nom.tam.fits.ImageData;
   import nom.tam.fits.RandomGroupsData;
   import nom.tam.fits.TableData;
   import nom.tam.fits.UndefinedData;
   
  /*
   * #%L
   * nom.tam FITS library
   * %%
   * Copyright (C) 1996 - 2024 nom-tam-fits
   * %%
   * This is free and unencumbered software released into the public domain.
   *
   * Anyone is free to copy, modify, publish, use, compile, sell, or
   * distribute this software, either in source code form or as a compiled
   * binary, for any purpose, commercial or non-commercial, and by any
   * means.
   *
   * In jurisdictions that recognize copyright laws, the author or authors
   * of this software dedicate any and all copyright interest in the
   * software to the public domain. We make this dedication for the benefit
   * of the public at large and to the detriment of our heirs and
   * successors. We intend this dedication to be an overt act of
   * relinquishment in perpetuity of all present and future rights to this
   * software under copyright law.
   *
   * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
   * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
   * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
   * IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
   * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
   * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
   * OTHER DEALINGS IN THE SOFTWARE.
   * #L%
   */
  
  /**
   * <p>
   * This data dictionary lists the 53 keywords currently defined in the FITS Standard.
   * </p>
   * <p>
   * See <a href=
   * "http://heasarc.gsfc.nasa.gov/docs/fcg/standard_dict.html">http://heasarc.gsfc.nasa.gov/docs/fcg/standard_dict.html</a>
   * </p>
   *
   * @author Richard van Nieuwenhoven
   */
  @SuppressWarnings("deprecation")
  public enum Standard implements IFitsHeader {
      /**
       * The value field shall contain a character string identifying who compiled the information in the data associated
       * with the key. This keyword is appropriate when the data originate in a published paper or are compiled from many
       * sources.
       */
      AUTHOR(SOURCE.RESERVED, HDU.ANY, VALUE.STRING, "author name(s)"),
      /**
       * The value field shall contain an integer. The absolute value is used in computing the sizes of data structures.
       * It shall specify the number of bits that represent a data value. RANGE: -64,-32,8,16,32
       * 
       * @see Bitpix
       */
      BITPIX(SOURCE.MANDATORY, HDU.ANY, VALUE.INTEGER, "bits per data element", //
              replaceable("header:bitpix", Object.class) //
      ),
  
      /**
       * This keyword shall be used only in primary array headers or IMAGE extension headers with positive values of
       * BITPIX (i.e., in arrays with integer data). Columns 1-8 contain the string, `BLANK ' (ASCII blanks in columns
       * 6-8). The value field shall contain an integer that specifies the representation of array values whose physical
       * values are undefined.
       */
      BLANK(SOURCE.RESERVED, HDU.IMAGE, VALUE.INTEGER, "value used for undefined array elements"),
  
      /**
       * Columns 1-8 contain ASCII blanks. This keyword has no associated value. Columns 9-80 may contain any ASCII text.
       * Any number of card images with blank keyword fields may appear in a key.
       */
      BLANKS("", SOURCE.RESERVED, HDU.ANY, VALUE.NONE, null),
  
      /**
       * This keyword may be used only in the primary key. It shall appear within the first 36 card images of the FITS
       * file. (Note: This keyword thus cannot appear if NAXIS is greater than 31, or if NAXIS is greater than 30 and the
       * EXTEND keyword is present.) Its presence with the required logical value of T advises that the physical block
       * size of the FITS file on which it appears may be an integral multiple of the logical record length, and not
       * necessarily equal to it. Physical block size and logical record length may be equal even if this keyword is
       * present or unequal if it is absent. It is reserved primarily to prevent its use with other meanings. Since the
       * issuance of version 1 of the standard, the BLOCKED keyword has been deprecated.
       *
       * @deprecated no blocksize other that 2880 may be used.
       */
      @Deprecated
      BLOCKED(SOURCE.RESERVED, HDU.PRIMARY, VALUE.LOGICAL, "Non-standard FITS block size"),
  
      /**
       * This keyword shall be used, along with the BZERO keyword, when the array pixel values are not the true physical
      * values, to transform the primary data array values to the true physical values they represent, using the
      * equation: physical_value = BZERO + BSCALE * array_value. The value field shall contain a floating point number
      * representing the coefficient of the linear term in the scaling equation, the ratio of physical value to array
      * value at zero offset. The default value for this keyword is 1.0.
      */
     BSCALE(SOURCE.RESERVED, HDU.IMAGE, VALUE.REAL, "data quantization scaling"),
 
     /**
      * The value field shall contain a character string, describing the physical units in which the quantities in the
      * array, after application of BSCALE and BZERO, are expressed. The units of all FITS key keyword values, with the
      * exception of measurements of angles, should conform with the recommendations in the IAU Style Manual. For angular
      * measurements given as floating point values and specified with reserved keywords, degrees are the recommended
      * units (with the units, if specified, given as 'deg').
      */
     BUNIT(SOURCE.RESERVED, HDU.IMAGE, VALUE.STRING, "data physical unit"),
 
     /**
      * This keyword shall be used, along with the BSCALE keyword, when the array pixel values are not the true physical
      * values, to transform the primary data array values to the true values using the equation: physical_value = BZERO
      * + BSCALE * array_value. The value field shall contain a floating point number representing the physical value
      * corresponding to an array value of zero. The default value for this keyword is 0.0.
      */
     BZERO(SOURCE.RESERVED, HDU.IMAGE, VALUE.REAL, "data quantization offset"),
 
     /**
      * The value field shall contain a floating point number giving the partial derivative of the coordinate specified
      * by the CTYPEn keywords with respect to the pixel index, evaluated at the reference point CRPIXn, in units of the
      * coordinate specified by the CTYPEn keyword. These units must follow the prescriptions of section 5.3 of the FITS
      * Standard.
      *
      * @see WCS#CDELTna
      */
     CDELTn(SOURCE.RESERVED, HDU.IMAGE, VALUE.REAL, "coordinate spacing along axis"),
 
     /**
      * This keyword shall have no associated value; columns 9-80 may contain any ASCII text. Any number of COMMENT card
      * images may appear in a key.
      */
     COMMENT(SOURCE.RESERVED, HDU.ANY, VALUE.NONE, null),
 
     /**
      * The CONTINUE keyword, when followed by spaces in columns 9 and 10 of the card image and a character string
      * enclosed in single quotes starting in column 11 or higher, indicates that the quoted string should be treated as
      * a continuation of the character string value in the previous key keyword. To conform to this convention, the
      * character string value on the previous keyword must end with the ampersand character ('&amp;'), but the ampersand
      * is not part of the value string and should be deleted before concatenating the strings together. The character
      * string value may be continued on any number of consecutive CONTINUE keywords, thus effectively allowing
      * arbitrarily long strings to be written as keyword values.
      */
     CONTINUE(SOURCE.RESERVED, HDU.ANY, VALUE.NONE, null),
 
     /**
      * This keyword is used to indicate a rotation from a standard coordinate system described by the CTYPEn to a
      * different coordinate system in which the values in the array are actually expressed. Rules for such rotations are
      * not further specified in the Standard; the rotation should be explained in comments. The value field shall
      * contain a floating point number giving the rotation angle in degrees between axis n and the direction implied by
      * the coordinate system defined by CTYPEn. In unit degrees.
      */
     CROTAn(SOURCE.RESERVED, HDU.IMAGE, VALUE.REAL, "[deg] coordinate axis rotation angle"),
 
     /**
      * The value field shall contain a floating point number, identifying the location of a reference point along axis
      * n, in units of the axis index. This value is based upon a counter that runs from 1 to NAXISn with an increment of
      * 1 per pixel. The reference point value need not be that for the center of a pixel nor lie within the actual data
      * array. Use comments to indicate the location of the index point relative to the pixel.
      * 
      * @see WCS#CRPIXna
      */
     CRPIXn(SOURCE.RESERVED, HDU.IMAGE, VALUE.REAL, "coordinate axis reference pixel"),
 
     /**
      * The value field shall contain a floating point number, giving the value of the coordinate specified by the CTYPEn
      * keyword at the reference point CRPIXn. Units must follow the prescriptions of section 5.3 of the FITS Standard.
      *
      * @see WCS#CRVALna
      */
     CRVALn(SOURCE.RESERVED, HDU.IMAGE, VALUE.REAL, "coordinate axis value at reference pixel"),
 
     /**
      * The value field shall contain a character string, giving the name of the coordinate represented by axis n.
      *
      * @see WCS#CTYPEna
      */
     CTYPEn(SOURCE.RESERVED, HDU.IMAGE, VALUE.STRING, "coordinate axis type / name"),
 
     /**
      * The value field shall always contain a floating point number, regardless of the value of BITPIX. This number
      * shall give the maximum valid physical value represented by the array, exclusive of any special values.
      */
     DATAMAX(SOURCE.RESERVED, HDU.IMAGE, VALUE.REAL, "maximum data value"),
 
     /**
      * The value field shall always contain a floating point number, regardless of the value of BITPIX. This number
      * shall give the minimum valid physical value represented by the array, exclusive of any special values.
      */
     DATAMIN(SOURCE.RESERVED, HDU.IMAGE, VALUE.REAL, "minimum data value"),
 
     /**
      * The date on which the HDU was created, in the format specified in the FITS Standard. The old date format was
      * 'yy/mm/dd' and may be used only for dates from 1900 through 1999. the new Y2K compliant date format is
      * 'yyyy-mm-dd' or 'yyyy-mm-ddTHH:MM:SS[.sss]'.
      * 
      * @see DateTime#DATE
      */
     DATE(SOURCE.RESERVED, HDU.ANY, VALUE.STRING, "date of file creation"),
 
     /**
      * The date of the observation, in the format specified in the FITS Standard. The old date format was 'yy/mm/dd' and
      * may be used only for dates from 1900 through 1999. The new Y2K compliant date format is 'yyyy-mm-dd' or
      * 'yyyy-mm-ddTHH:MM:SS[.sss]'.
      * 
      * @see DateTime#DATE_OBS
      */
     DATE_OBS("DATE-OBS", SOURCE.RESERVED, HDU.ANY, VALUE.STRING, "date of the observation"),
 
     /**
      * This keyword has no associated value. Columns 9-80 shall be filled with ASCII blanks.
      */
     END(SOURCE.MANDATORY, HDU.ANY, VALUE.NONE, null),
 
     /**
      * The value field shall contain a floating point number giving the equinox in years for the celestial coordinate
      * system in which positions are expressed. Starting with Version 1, the Standard has deprecated the use of the
      * EPOCH keyword and thus it shall not be used in FITS files created after the adoption of the standard; rather, the
      * EQUINOX keyword shall be used.
      *
      * @deprecated Deprecated by the FITS standard in favor of {@link #EQUINOX}.
      */
     EPOCH(SOURCE.RESERVED, HDU.ANY, VALUE.REAL, "[yr] equinox of celestial coordinate system"),
 
     /**
      * The value field shall contain a floating point number giving the equinox in years for the celestial coordinate
      * system in which positions are expressed. This version of the keyword does not support alternative coordinate
      * systems.
      * 
      * @see WCS#EQUINOXa
      */
     EQUINOX(SOURCE.RESERVED, HDU.ANY, VALUE.REAL, "[yr] equinox of celestial coordinate system"),
 
     /**
      * If the FITS file may contain extensions, a card image with the keyword EXTEND and the value field containing the
      * logical value T must appear in the primary key immediately after the last NAXISn card image, or, if NAXIS=0, the
      * NAXIS card image. The presence of this keyword with the value T in the primary key does not require that
      * extensions be present.
      */
     EXTEND(SOURCE.INTEGRAL, HDU.PRIMARY, VALUE.LOGICAL, "allow extensions"),
 
     /**
      * The value field shall contain an integer, specifying the level in a hierarchy of extension levels of the
      * extension key containing it. The value shall be 1 for the highest level; levels with a higher value of this
      * keyword shall be subordinate to levels with a lower value. If the EXTLEVEL keyword is absent, the file should be
      * treated as if the value were 1. This keyword is used to describe an extension and should not appear in the
      * primary key.RANGE: [1:] DEFAULT: 1
      */
     EXTLEVEL(SOURCE.RESERVED, HDU.ANY, VALUE.INTEGER, "hierarchical level of the extension"),
 
     /**
      * The value field shall contain a character string, to be used to distinguish among different extensions of the
      * same type, i.e., with the same value of XTENSION, in a FITS file. This keyword is used to describe an extension
      * and but may appear in the primary header also.
      */
     EXTNAME(SOURCE.RESERVED, HDU.ANY, VALUE.STRING, "HDU name"),
 
     /**
      * The value field shall contain an integer, to be used to distinguish among different extensions in a FITS file
      * with the same type and name, i.e., the same values for XTENSION and EXTNAME. The values need not start with 1 for
      * the first extension with a particular value of EXTNAME and need not be in sequence for subsequent values. If the
      * EXTVER keyword is absent, the file should be treated as if the value were 1. This keyword is used to describe an
      * extension and should not appear in the primary key.RANGE: [1:] DEFAULT: 1
      */
     EXTVER(SOURCE.RESERVED, HDU.ANY, VALUE.INTEGER, "HDU version"),
 
     /**
      * The value field shall contain an integer that shall be used in any way appropriate to define the data structure,
      * consistent with Eq. 5.2 in the FITS Standard. This keyword originated for use in FITS Random Groups where it
      * specifies the number of random groups present. In most other cases this keyword will have the value 1.
      */
     GCOUNT(SOURCE.MANDATORY, HDU.ANY, VALUE.INTEGER, "group count",
             replaceable("randomgroupsdata:groups", RandomGroupsData.class), //
             replaceable("undefineddata:groups", UndefinedData.class), //
             replaceable("header:groups", RandomGroupsData.class) //
     ),
 
     /**
      * The value field shall contain the logical constant T. The value T associated with this keyword implies that
      * random groups records are present.
      */
     GROUPS(SOURCE.MANDATORY, HDU.GROUPS, VALUE.LOGICAL, "random groups data", //
             replaceable("randomgroupsdata:groups", RandomGroupsData.class) //
     ),
 
     /**
      * This keyword shall have no associated value; columns 9-80 may contain any ASCII text. The text should contain a
      * history of steps and procedures associated with the processing of the associated data. Any number of HISTORY card
      * images may appear in a key.
      */
     HISTORY(SOURCE.RESERVED, HDU.ANY, VALUE.NONE, "processing history of the data"),
 
     /**
      * The value field shall contain a character string identifying the instrument used to acquire the data associated
      * with the key.
      */
     INSTRUME(SOURCE.RESERVED, HDU.ANY, VALUE.STRING, "name of instrument"),
 
     /**
      * The value field shall contain a non-negative integer no greater than 999, representing the number of axes in the
      * associated data array. A value of zero signifies that no data follow the key in the HDU. In the context of FITS
      * 'TABLE' or 'BINTABLE' extensions, the value of NAXIS is always 2.RANGE: [0:999]
      */
     NAXIS(SOURCE.MANDATORY, HDU.ANY, VALUE.INTEGER, "dimensionality of data"),
 
     /**
      * The value field of this indexed keyword shall contain a non-negative integer, representing the number of elements
      * along axis n of a data array. The NAXISn must be present for all values n = 1,...,NAXIS, and for no other values
      * of n. A value of zero for any of the NAXISn signifies that no data follow the key in the HDU. If NAXIS is equal
      * to 0, there should not be any NAXISn keywords.RANGE: [0:]
      */
     NAXISn(SOURCE.MANDATORY, HDU.ANY, VALUE.INTEGER, "n'th data dimension", //
             replaceable("tablehdu:naxis1", TableData.class, "Size of table row in bytes"), //
             replaceable("tablehdu:naxis2", TableData.class, "Number of table rows"),
             replaceable("header:naxis2", Object.class)),
 
     /**
      * The value field shall contain a character string giving a name for the object observed.
      */
     OBJECT(SOURCE.RESERVED, HDU.ANY, VALUE.STRING, "name of observed object"),
 
     /**
      * The value field shall contain a character string identifying who acquired the data associated with the key.
      */
     OBSERVER(SOURCE.RESERVED, HDU.ANY, VALUE.STRING, "observer(s) who acquired the data"),
 
     /**
      * The value field shall contain a character string identifying the organization or institution responsible for
      * creating the FITS file.
      */
     ORIGIN(SOURCE.RESERVED, HDU.ANY, VALUE.STRING, "organization responsible for the data"),
 
     /**
      * The value field shall contain an integer that shall be used in any way appropriate to define the data structure,
      * consistent with Eq. 5.2 in the FITS Standard. This keyword was originated for use with FITS Random Groups and
      * represented the number of parameters preceding each group. It has since been used in 'BINTABLE' extensions to
      * represent the size of the data heap following the main data table. In most other cases its value will be zero.
      */
     PCOUNT(SOURCE.MANDATORY, HDU.ANY, VALUE.INTEGER, "associated parameter count", //
             replaceable("binarytable:pcount", TableData.class, "heap size in bytes"),
             replaceable("randomgroups:pcount", RandomGroupsData.class, "parameter values per group"),
             replaceable("undefineddata:pcount", UndefinedData.class), replaceable("header:pcount", Object.class)),
 
     /**
      * This keyword is reserved for use within the FITS Random Groups structure. This keyword shall be used, along with
      * the PZEROn keyword, when the nth FITS group parameter value is not the true physical value, to transform the
      * group parameter value to the true physical values it represents, using the equation, physical_value = PZEROn +
      * PSCALn * group_parameter_value. The value field shall contain a floating point number representing the
      * coefficient of the linear term, the scaling factor between true values and group parameter values at zero offset.
      * The default value for this keyword is 1.0.
      */
     PSCALn(SOURCE.INTEGRAL, HDU.GROUPS, VALUE.REAL, "parameter quantization scaling"),
 
     /**
      * This keyword is reserved for use within the FITS Random Groups structure. The value field shall contain a
      * character string giving the name of parameter n. If the PTYPEn keywords for more than one value of n have the
      * same associated name in the value field, then the data value for the parameter of that name is to be obtained by
      * adding the derived data values of the corresponding parameters. This rule provides a mechanism by which a random
      * parameter may have more precision than the accompanying data array elements; for example, by summing two 16-bit
      * values with the first scaled relative to the other such that the sum forms a number of up to 32-bit precision.
      */
     PTYPEn(SOURCE.INTEGRAL, HDU.GROUPS, VALUE.STRING, "name of random groups parameter"),
 
     /**
      * This keyword is reserved for use within the FITS Random Groups structure. This keyword shall be used, along with
      * the PSCALn keyword, when the nth FITS group parameter value is not the true physical value, to transform the
      * group parameter value to the physical value. The value field shall contain a floating point number, representing
      * the true value corresponding to a group parameter value of zero. The default value for this keyword is 0.0. The
      * transformation equation is as follows: physical_value = PZEROn + PSCALn * group_parameter_value.DEFAULT: 0.0
      */
     PZEROn(SOURCE.INTEGRAL, HDU.GROUPS, VALUE.REAL, "parameter quantization offset"),
 
     /**
      * Coordinate reference frame of major/minor axes.If absent the default value is 'FK5'. This version of the keyword
      * does not support alternative coordinate systems.
      * 
      * @see WCS#RADESYSa
      */
     RADESYS(SOURCE.RESERVED, HDU.ANY, VALUE.STRING, "celestial coordinate reference frame"),
 
     /**
      * Coordinate reference frame of major/minor axes (generic).
      *
      * @deprecated Deprecated in the current FITS satndard, use {@link #RADESYS} instead.
      */
     RADECSYS(SOURCE.RESERVED, HDU.ANY, VALUE.STRING, "celestial coordinate reference frame"),
 
     /**
      * [Hz] Rest frequency of observed spectral line.
      * 
      * @since 1.19
      * 
      * @see   WCS#RESTFRQa
      */
     RESTFRQ(SOURCE.RESERVED, HDU.IMAGE, VALUE.REAL, "[Hz] line rest frequency"),
 
     /**
      * [Hz] Rest frequeny of observed spectral line (generic).
      *
      * @deprecated Deprecated in the current FITS standard, use {@link #RESTFRQ} instead.
      */
     RESTFREQ(SOURCE.RESERVED, HDU.ANY, VALUE.REAL, "[Hz] observed line rest frequency"),
 
     /**
      * The value field shall contain a character string citing a reference where the data associated with the key are
      * published.
      */
     REFERENC(SOURCE.RESERVED, HDU.ANY, VALUE.STRING, "bibliographic reference"),
 
     /**
      * The SIMPLE keyword is required to be the first keyword in the primary key of all FITS files. The value field
      * shall contain a logical constant with the value T if the file conforms to the standard. This keyword is mandatory
      * for the primary key and is not permitted in extension headers. A value of F signifies that the file does not
      * conform to this standard.
      */
     SIMPLE(SOURCE.MANDATORY, HDU.PRIMARY, VALUE.LOGICAL, "primary HDU", //
             replaceable("header:simple", Object.class, "Java FITS: " + new java.util.Date()) //
     ),
 
     /**
      * The value field of this indexed keyword shall contain an integer specifying the column in which field n starts in
      * an ASCII TABLE extension. The first column of a row is numbered 1.RANGE: [1:]
      */
     TBCOLn(SOURCE.MANDATORY, HDU.ASCII_TABLE, VALUE.INTEGER, "column byte offset", //
             replaceable("asciitable:tbcolN", AsciiTable.class) //
     ),
 
     /**
      * The value field of this indexed keyword shall contain a character string describing how to interpret the contents
      * of field n as a multidimensional array, providing the number of dimensions and the length along each axis. The
      * form of the value is not further specified by the Standard. A proposed convention is described in Appendix B.2 of
      * the FITS Standard in which the value string has the format '(l,m,n...)' where l, m, n,... are the dimensions of
      * the array.
      */
     TDIMn(SOURCE.RESERVED, HDU.BINTABLE, VALUE.STRING, "dimensionality of column array elements", //
             replaceable("binarytable:tdimN", BinaryTable.class) //
     ),
 
     /**
      * The value field of this indexed keyword shall contain a character string describing the format recommended for
      * the display of the contents of field n. If the table value has been scaled, the physical value shall be
      * displayed. All elements in a field shall be displayed with a single, repeated format. For purposes of display,
      * each byte of bit (type X) and byte (type B) arrays is treated as an unsigned integer. Arrays of type A may be
      * terminated with a zero byte. Only the format codes in Table 8.6, discussed in section 8.3.4 of the FITS Standard,
      * are permitted for encoding. The format codes must be specified in upper case. If the Bw.m, Ow.m, and Zw.m formats
      * are not readily available to the reader, the Iw.m display format may be used instead, and if the ENw.d and ESw.d
      * formats are not available, Ew.d may be used. The meaning of this keyword is not defined for fields of type P in
      * the Standard but may be defined in conventions using such fields.
      */
     TDISPn(SOURCE.RESERVED, HDU.TABLE, VALUE.STRING, "column display format"),
 
     /**
      * The value field shall contain a character string identifying the telescope used to acquire the data associated
      * with the key.
      */
     TELESCOP(SOURCE.RESERVED, HDU.ANY, VALUE.STRING, "name of telescope / observatory"),
 
     /**
      * The value field shall contain a non-negative integer representing the number of fields in each row of a 'TABLE'
      * or 'BINTABLE' extension. The maximum permissible value is 999. RANGE: [0:999]
      */
     TFIELDS(SOURCE.MANDATORY, HDU.TABLE, VALUE.INTEGER, "number of columns in the table"),
 
     /**
      * The value field of this indexed keyword shall contain a character string describing the format in which field n
      * is encoded in a 'TABLE' or 'BINTABLE' extension.
      */
     TFORMn(SOURCE.MANDATORY, HDU.TABLE, VALUE.STRING, "column data format", //
             replaceable("asciitable:tformN", AsciiTable.class), //
             replaceable("binarytable:tformN", BinaryTable.class) //
     ),
 
     /**
      * The value field of this keyword shall contain an integer providing the separation, in bytes, between the start of
      * the main data table and the start of a supplemental data area called the heap. The default value shall be the
      * product of the values of NAXIS1 and NAXIS2. This keyword shall not be used if the value of PCOUNT is zero. A
      * proposed application of this keyword is presented in Appendix B.1 of the FITS Standard.
      */
     THEAP(SOURCE.INTEGRAL, HDU.BINTABLE, VALUE.INTEGER, "heap byte offset", //
             replaceable("binarytable:theap", BinaryTable.class) //
     ),
 
     /**
      * In ASCII 'TABLE' extensions, the value field for this indexed keyword shall contain the character string that
      * represents an undefined value for field n. The string is implicitly blank filled to the width of the field. In
      * binary 'BINTABLE' table extensions, the value field for this indexed keyword shall contain the integer that
      * represents an undefined value for field n of data type B, I, or J. The keyword may not be used in 'BINTABLE'
      * extensions if field n is of any other data type.
      */
     TNULLn(SOURCE.INTEGRAL, HDU.TABLE, VALUE.ANY, "column value for undefined elements"),
 
     /**
      * This indexed keyword shall be used, along with the TZEROn keyword, when the quantity in field n does not
      * represent a true physical quantity. The value field shall contain a floating point number representing the
      * coefficient of the linear term in the equation, physical_value = TZEROn + TSCALn * field_value, which must be
      * used to compute the true physical value of the field, or, in the case of the complex data types C and M, of the
      * real part of the field with the imaginary part of the scaling factor set to zero. The default value for this
      * keyword is 1.0. This keyword may not be used if the format of field n is A, L, or X.DEFAULT: 1.0
      */
     TSCALn(SOURCE.RESERVED, HDU.TABLE, VALUE.REAL, "column quantization scaling"),
 
     /**
      * The value field for this indexed keyword shall contain a character string, giving the name of field n. It is
      * recommended that only letters, digits, and underscore (hexadecimal code 5F, ('_') be used in the name. String
      * comparisons with the values of TTYPEn keywords should not be case sensitive. The use of identical names for
      * different fields should be avoided.
      */
     TTYPEn(SOURCE.RESERVED, HDU.TABLE, VALUE.STRING, "column name"),
 
     /**
      * The value field shall contain a character string describing the physical units in which the quantity in field n,
      * after any application of TSCALn and TZEROn, is expressed. The units of all FITS key keyword values, with the
      * exception of measurements of angles, should conform with the recommendations in the IAU Style Manual. For angular
      * measurements given as floating point values and specified with reserved keywords, degrees are the recommended
      * units (with the units, if specified, given as 'deg').
      */
     TUNITn(SOURCE.RESERVED, HDU.TABLE, VALUE.STRING, "column physical unit"),
 
     /**
      * This indexed keyword shall be used, along with the TSCALn keyword, when the quantity in field n does not
      * represent a true physical quantity. The value field shall contain a floating point number representing the true
      * physical value corresponding to a value of zero in field n of the FITS file, or, in the case of the complex data
      * types C and M, in the real part of the field, with the imaginary part set to zero. The default value for this
      * keyword is 0.0. This keyword may not be used if the format of field n is A, L, or X.DEFAULT: 0.0
      */
     TZEROn(SOURCE.RESERVED, HDU.TABLE, VALUE.REAL, "column quantization offset"),
 
     /**
      * The value field of this indexed keyword shall contain a floating point number specifying the maximum valid
      * physical value represented in column n of the table, exclusive of any special values. This keyword may only be
      * used in 'TABLE' or 'BINTABLE' extensions and is analogous to the DATAMAX keyword used for FITS images.
      * 
      * @since 1.19
      */
     TDMAXn(SOURCE.RESERVED, HDU.TABLE, VALUE.REAL, "maximum value in the column"),
 
     /**
      * The value field of this indexed keyword shall contain a floating point number specifying the minimum valid
      * physical value represented in column n of the table, exclusive of any special values. This keyword may only be
      * used in 'TABLE' or 'BINTABLE' extensions and is analogous to the DATAMIN keyword used for FITS images.
      * 
      * @since 1.19
      */
     TDMINn(SOURCE.RESERVED, HDU.TABLE, VALUE.REAL, "minimum value in the column"),
 
     /**
      * The value field of this indexed keyword shall contain a floating point number specifying the upper bound of the
      * legal range of physical values that may be represented in column n of the table. The column may contain values
      * that are greater than this legal maximum value but the interpretation of such values is not defined here. The
      * value of this keyword is typically used as the maxinum value when constructing a histogram of the values in the
      * column. This keyword may only be used in 'TABLE' or 'BINTABLE' extensions.
      * 
      * @since 1.19
      */
     TLMAXn(SOURCE.RESERVED, HDU.TABLE, VALUE.REAL, "maximum legal value in the column"),
 
     /**
      * The value field of this indexed keyword shall contain a floating point number specifying the lower bound of the
      * legal range of physical values that may be represented in column n of the table. The column may contain values
      * that are less than this legal minimum value but the interpretation of such values is not defined here. The value
      * of this keyword is typically used as the mininum value when constructing a histogram of the values in the column.
      * This keyword may only be used in 'TABLE' or 'BINTABLE' extensions.
      * 
      * @since 1.19
      */
     TLMINn(SOURCE.RESERVED, HDU.TABLE, VALUE.REAL, "minimum legal value in the column"),
 
     /**
      * The value field shall contain a character string giving the name of the extension type. This keyword is mandatory
      * for an extension key and must not appear in the primary key. For an extension that is not a standard extension,
      * the type name must not be the same as that of a standard extension.
      */
     XTENSION(SOURCE.MANDATORY, HDU.EXTENSION, VALUE.STRING, "HDU extension type",
             replaceable("imagedata:xtension", ImageData.class, "image HDU"), //
             replaceable("binarytable:xtension", BinaryTable.class, "binary table HDU"), //
             replaceable("asciitable:xtension", AsciiTable.class, "ASCII table HDU"), //
             replaceable("undefineddata:xtension", UndefinedData.class), //
             replaceable("header:xtension", Object.class) //
     ),
 
     /**
      * If set to <code>true</code>, it indicates that the HDU should inherit all non-confliucting keywords from the
      * primary HDU.
      * 
      * @since 1.19
      */
     INHERIT(SOURCE.RESERVED, HDU.EXTENSION, VALUE.LOGICAL, "inherit primary header entries");
 
     private static final ThreadLocal<Class<?>> COMMENT_CONTEXT = new ThreadLocal<>();
 
     /**
      * A shorthand for {@link #NAXISn}<code>.n(1)</code>, that is the regular dimension along the first, fastest FITS
      * array index (this is the same as the last dimension of Java arrays).
      */
     public static final IFitsHeader NAXIS1 = NAXISn.n(1);
 
     /**
      * A shorthand for {@link #NAXISn}<code>.n(2)</code>, that is the regular dimension along the second fastest FITS
      * array index (this is the same as the one before the last dimension of Java arrays).
      */
     public static final IFitsHeader NAXIS2 = NAXISn.n(2);
 
     /**
      * The value of the XTENSION keword in case of a binary table.
      */
     public static final String XTENSION_ASCIITABLE = "TABLE";
 
     /**
      * The value of the XTENSION keword in case of a binary table.
      */
     public static final String XTENSION_BINTABLE = "BINTABLE";
 
     /**
      * The value of the XTENSION keword in case of an image.
      */
     public static final String XTENSION_IMAGE = "IMAGE";
 
     private final StandardCommentReplacement[] commentReplacements;
 
     private final FitsKey key;
 
     Standard(SOURCE status, HDU hdu, VALUE valueType, String comment, StandardCommentReplacement... replacements) {
         this(null, status, hdu, valueType, comment, replacements);
     }
 
     Standard(String headerName, SOURCE status, HDU hdu, VALUE valueType, String comment,
             StandardCommentReplacement... replacements) {
         key = new FitsKey(headerName == null ? name() : headerName, status, hdu, valueType, comment);
         commentReplacements = replacements;
         FitsKey.registerStandard(this);
     }
 
     @Override
     public final FitsKey impl() {
         return key;
     }
 
     @Override
     public String comment() {
         Class<?> contextClass = COMMENT_CONTEXT.get();
         if (contextClass == null) {
             contextClass = Object.class;
         }
         for (StandardCommentReplacement stdCommentReplacement : commentReplacements) {
             if (stdCommentReplacement.getContext().isAssignableFrom(contextClass)) {
                 if (stdCommentReplacement.getComment() != null) {
                     return stdCommentReplacement.getComment();
                 }
             }
         }
         return key.comment();
     }
 
     /**
      * @deprecated       (<i>for internal use</i>) Using {@link nom.tam.fits.HeaderCard#setComment(String)} after
      *                       creating a header card with this keyword provides a more transparent way of setting
      *                       context-specific comments. This convoluted approach is no longer supported and will be
      *                       removed in the future.
      * 
      * @param      clazz Usually a subclass of <code>nom.tam.fits.Data</code>.
      * 
      * @see              nom.tam.fits.HeaderCard#setComment(String)
      */
     public static void context(Class<?> clazz) {
         COMMENT_CONTEXT.set(clazz);
     }
 
     /**
      * scan for a comment with the specified reference key.
      *
      * @param      commentKey the reference key
      *
      * @return                the comment for the reference key
      * 
      * @deprecated            (<i>)for internal use</i>)
      */
     public String getCommentByKey(String commentKey) {
         for (StandardCommentReplacement commentReplacement : commentReplacements) {
             if (commentReplacement.getRef().equals(commentKey)) {
                 String foundcommentReplacement = commentReplacement.getComment();
                 if (foundcommentReplacement == null) {
                     return comment();
                 }
                 return foundcommentReplacement;
             }
         }
         return null;
     }
 
     /**
      * set the comment for the specified reference key.
      *
      * @param      commentKey the reference key
      * @param      value      the comment to set when the fits key is used.
      * 
      * @deprecated            (<i>)for internal use</i>)
      */
     public void setCommentByKey(String commentKey, String value) {
         for (StandardCommentReplacement commentReplacement : commentReplacements) {
             if (commentReplacement.getRef().equals(commentKey)) {
                 commentReplacement.setComment(value);
                 return;
             }
         }
     }
 
     private static StandardCommentReplacement replaceable(String string, Class<?> clazz) {
         return new StandardCommentReplacement(string, clazz);
     }
 
     private static StandardCommentReplacement replaceable(String string, Class<?> clazz, String comment) {
         return new StandardCommentReplacement(string, clazz, comment);
     }
 
     /**
      * Returns the standard FITS keyword that matches the specified actual key.
      * 
      * @param  key The key as it may appear in a FITS header, e.g. "CTYPE1A"
      * 
      * @return     The standard FITS keyword/pattern that matches, e.g. {@link WCS#CTYPEna}.
      * 
      * @see        IFitsHeader#extractIndices(String)
      * 
      * @since      1.19
      */
     public static IFitsHeader match(String key) {
         return FitsKey.matchStandard(key);
     }
 
 }