let p, q be ZF-formula; :: thesis: for M being non empty set
for v being Function of VAR ,M holds
( M,v |= (p <=> q) => (q => p) & M |= (p <=> q) => (q => p) )
let M be non empty set ; :: thesis: for v being Function of VAR ,M holds
( M,v |= (p <=> q) => (q => p) & M |= (p <=> q) => (q => p) )
let v be Function of VAR ,M; :: thesis: ( M,v |= (p <=> q) => (q => p) & M |= (p <=> q) => (q => p) )

A1: now

let v be Function of VAR ,M; :: thesis: M,v |= (p <=> q) => (q => p)
( M,v |= p <=> q implies M,v |= q => p ) by ZF_MODEL:15;
hence M,v |= (p <=> q) => (q => p) by ZF_MODEL:18; :: thesis: verum

end;

hence M,v |= (p <=> q) => (q => p) ; :: thesis: M |= (p <=> q) => (q => p)
let v be Function of VAR ,M; :: according to ZF_MODEL:def 5 :: thesis: M,v |= (p <=> q) => (q => p)
thus M,v |= (p <=> q) => (q => p) by A1; :: thesis: verum