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.