+ It is forbidden to undo a "move + action". For example here, white could
+ push back the black bishop on g7 but not return to d4 then.
+
+figure.diagram-container
+ .diagram.diag12
+ | fen:rnbqk1nr/ppppppbp/6p1/8/3B4/1P6/P1PPPPPP/RN1QKBNR:
+ .diagram.diag22
+ | fen:rnbqk1nr/pppppp1p/6p1/8/3b4/1P6/PBPPPPPP/RN1QKBNR:
+ figcaption Pushing the d4 bishop to b2 (left: before, right: after).
+
+p.
+ Castling is possible as long as the king and rook have not moved and