
theorem Th12:
  for f1,f2 being complex-valued Function holds dom (f1-f2) = dom
  f1 /\ dom f2
proof
  let f1,f2 be complex-valued Function;
  thus dom (f1-f2) = dom f1 /\ dom -f2 by Def1
    .= dom f1 /\ dom f2 by Def5;
end;
