# Sed script that inserts the file called HEADER before the header entry.
  #
  # At each occurrence of a line starting with "msgid ", we execute the following
  # commands. At the first occurrence, insert the file. At the following
  # occurrences, do nothing. The distinction between the first and the following
  # occurrences is achieved by looking at the hold space.
  /^msgid /{
  x
  # Test if the hold space is empty.
  s/m/m/
  ta
  # Yes it was empty. First occurrence. Read the file.
  r HEADER
  # Output the file's contents by reading the next line. But don't lose the
  # current line while doing this.
  g
  N
  bb
  :a
  # The hold space was nonempty. Following occurrences. Do nothing.
  x
  :b
  }