Challenge No. 18 Minimal number of units to force a move to be legal or illegal. Only castling and en passant moves can be proven legal or illegal. Under this challenge, we are looking for positions (not necessarily DR restricted) where only under DR is the move provably legal or illegal. The idea is to do this with the minimum number of units. There are basically 6 cases: legality or illegality of en passant, or White or Black castling. The following 3 examples are known: 4k2r/1p4p1/1P5p/KP6/PP6/8/8/8 w - - id #073 (5+5) Noam Elkies. White to move. Castling rights?; 8/8/8/Pp6/pp6/kp6/1p6/1K6 w - - id #074 (2+6) Andrew Buchanan version Noam Elkies. En passant OK?; [Putting a bPa6 leaves a best known value for challenge 19: minimal units for "White to move. Stalemate?"] 7k/6pP/6Pp/KpP4P/1P6/8/8/8 w - - id #058 (6+4) White to move. En passant OK?; [This one happens to be a DR restricted position also, but that is irrelevant for this challenge.] So there is lots of scope with this one! It's not important to prove that your answer is minimal. Just to try to come up with the smallest example you can.