scheme Extensionality { X,Y() -> set, P[set] } : X() = Y() provided A1: for x holds x in X() iff P[x] and A2: for x holds x in Y() iff P[x] proof A3: x in X() implies x in Y() proof assume x in X(); then P[x] by A1; hence x in Y() by A2; end; x in Y() implies x in X() proof assume x in Y(); then P[x] by A2; hence x in X() by A1; end; hence thesis by TARSKI:2,A3; end;証明は,