reserve p,p1,p2,q,r,F,G,G1,G2,H,H1,H2 for ZF-formula,
  x,x1,x2,y,y1,y2,z,z1,z2,s,t for Variable,
  a,X for set;
reserve M for non empty set,
  m,m9 for Element of M,
  v,v9 for Function of VAR,M;
reserve i,j for Element of NAT;

theorem
  M,v |= (p <=> q) => (p => q) & M |= (p <=> q) => (p => q)
proof
A1: now
    let v;
    M,v |= p <=> q implies M,v |= p => q by ZF_MODEL:15;
    hence M,v |= (p <=> q) => (p => q) by ZF_MODEL:18;
  end;
  hence M,v |= (p <=> q) => (p => q);
  let v;
  thus thesis by A1;
end;
