The purpose of the prolog.ps file is to set up non-generic fonts. Applications use these pre-defined PostScript font names for printing. The prolog file must define at least the following font names for Desk Set Calendar manager and mp.
LC_Times-Roman
LC_Times-Bold
LC_Helvetica
LC_Helvetica-Bold
LC_Courier
LC_Helvetica-BoldOblique
LC_Times-Italic
These fonts need to be able to print the local character set in the following example usage of those fonts:
100 100 moveto
/LC_Times-Roman findfont 24 scale font setfont
(Any text string in your locale) show