Let t on final leg in Betza notation forbid checking

Let t on final leg in Betza notation forbid checking

To allow approximate definition of pieces that are too difficult to
describe exactly, we can revoke their capture rights on Kings,
to prevent imagined moves that happen to threaten a King forces the
opponent to evade this imagined check, making most of his moves illegal.
Then the piece can be safely replaced by an upward-compatible one,
leaving rejection of the extra moves to the engine.