The common prolog file is mp.common.ps.
Every other page layout prolog file needs to include this file.
The mp.common.ps file resides in the /usr/lib/lp/locale/C/mp/ directory. This file contains a PostScript routine to re-encode a font from the standard encoding to the ISO 8859–1 encoding. The .reencodeISO routine is called from the print layout prolog files to change encoding of the fonts. Usually this prolog file does not need any customization. If you create your own prolog file, set the environment variable MP_PROLOGUE to point to the directory that contains the modified prolog files.