theorem Th26:
  for x being Variable of f for t being INT-Expression of A,f
  holds f.(s, x:=t).x = t.s & for z st z <> x holds f.(s, x:=t).z = s.z
proof
  let x be Variable of f;
  let t be INT-Expression of A,f;
  (^x).s = x;
  hence thesis by Th24;
end;
