      fields (file) (header) [-X N] [-comment N line]

      Add header entries to the given file.  The header information to be
      inserted is read from the provided header file, which contains
      abbreviated FITS header lines:  each line must consist of a keyword
      and data value, following the FITS conventions, but lines are broken
      with returns, and there are no required keys or minimum number of
      lines.  By default, the primary header unit is used, but if the -X
      flag is used, the Nth extension is used instead, where 0 corresponds
      to the first extension after the PHU.  If the -comment option is
      provided, the Nth comment is replaced with the provided line.  By
      default, this function will search for blank lines and overwrite the
      data in-situ.  However, if it is necessary to extend the header, a
      new file will be written.