以下は,定理
theorem BOOLE'1: :: BOOLE'1: X meets Y iff ex x st x in X & x in Y proof hereby assume X meets Y; then X /\ Y <> {} by Def7; then consider x such that A1: x in X /\ Y by Def1; take x; thus x in X & x in Y by Def3,A1; end; given x such that A2: x in X & x in Y; x in X /\ Y by Def3,A2; then X /\ Y <> {} by Def1; hence thesis by Def7; end;証明は,