[Retros] En-passant chess

[Noam Elkies]

Franco wrote:


> Anyway, when you'll compose the first En Passant Chess problem,

> I'll glad to enjoy it.


Not so much a composition as a translation of an old puzzle:

  White Royal Pawn h7, Black Royal Rook a1 (1+1)
  Checkless En Passant Chess: direct stalemate in 13

If I count right there are 924 full-length variations...
How many distinct mutual Zugzwang positions?

NDE