theorem Th93:
  for f being Function, a, b being object holds (f +* (a .--> b)).a =
  b
proof
  let f be Function, a, b be object;
  a in dom (a .--> b) by TARSKI:def 1;
  hence (f +* (a .--> b)).a = (a .--> b).a by FUNCT_4:13
    .= b by FUNCOP_1:72;
end;
