How can I set up en passant in the Popeye engine? I want to solve a problem with Thomas Maeder's excellent Popeye engine, and to specify that a possible en passant in the diagram is ok. How can I do this please? The documentation seems opaque. Thanks so much!

See https://github.com/thomas-maeder/popeye/releases/tag/v4.81 

EDIT: Outside chess, I see stackexchange used widely to ask technical questions about software use (indeed use it myself like this sometimes), and I think this is fair use in chess too.
The github page lists several help files in various languages: English, French and German. These are text files which can be downloaded. English is clearly not Thomas Maeder's mother tongue, I'm guessing German. Perhaps the German help file is clearer than the English.

As far as I can tell from the English file there is a long list of options you can pass to the program to define the problem you want it to solve. For en passant it defines the "EnPassant" option as follows:


  EnPassant  - Must be followed by 3-4 squares which indicate the  pawn
  move just played:
  
  
  departure square   
  avoided squares   
  arrival square