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I have looked at 'chess2.txt' as you recommended, and rewrote my
Squirrel
Chess preset to use it (because this was easier than Wildebeest Chess).
Also I have experimented and looked at some other presets.
Thereby I have found some bugs:

The operator sign calculates the sign of the operand decremented by 1.

In the preset for orthodox chess (and maybe in some others too) it is not
tested if a rook to castle with has been captured. One should add
  unsetflag dest;
at an appropriate place in the code.

In the file 'chess2.txt':

In function p: checkatwostep must have the arguments 0 -1 0 -1;

Functions p and P allow captures to occupied squares only (except
enpassant). Thus they are not suited to test if an empty square is
attacked, and the subroutine 'castle' doesn't work correctly. It calls
'checked' to test for castling out of or through check, which in turn
uses p and P.

In sub stalemated: If a royal pawn captures en-passant, the captured pawn
isn't removed. (also, 'store' and 'restore' are called more often
than necessary.)

In sub 'enpassant': The line
  set #pe join filename #to filename #from;
should be replaced by
  local pe;
  set pe join filename #to rankname #from;

In sub castle: near the end there should be 'subargs' instead of
'subarg'.



Also I have some questions:

Is there a difference between numbers and strings, or are numbers just
strings consisting of digits only?
In the following code:
set wpr 2;
if == rankname #from #wpr: ... endif;
either rankname gives a number if the ranks are labelled with numbers, or
numbers are strings, or numbers are implicitly converted to strings (or
vice versa).

Wouldn't it be reasonable to check the legality of the last move only?
In
all presets I have seen the legality of every move is checked, but if a
preset rejects all illegal moves, all moves except maybe the last must be
legal, if I don't overlook something.


BTW: I plan to write a 'chess3.txt' include file which is more flexible
and general, first to practice writing GAME Code and second because it
might be of use for you and others. Before I begin, I should know if it
can be a mistake to thest the legality of the last move only. This
would improve efficiency in long games.