
theorem
  for f1,f2 being complex-valued Function
  for c being object st c in dom (f1-f2) holds (f1-f2).c = f1.c - f2.c
proof
  let f1,f2 be complex-valued Function;
  let c be object;
  assume c in dom (f1-f2);
  hence (f1-f2).c = f1.c+(-f2).c by Def1
    .= f1.c-f2.c by Th8;
end;
