
theorem
  for a,b be Element of F_Complex st Im a = 0 & Im b = 0 holds Im (a*b) = 0
proof
  let a,b be Element of F_Complex;
  assume Im a = 0 & Im b = 0;
  hence Im (a*b) = Re b * 0 by Th10
    .= 0;
end;
