
theorem Th3:
  for x1,y1 be Element of F_Complex for x2,y2 be Complex st
  x1 = x2 & y1 = y2 holds x1 - y1 = x2 - y2
proof
  let x1,y1 be Element of F_Complex;
  let x2,y2 be Complex;
  assume that
A1: x1 = x2 and
A2: y1 = y2;
  - y1 = -y2 by A2,Th2;
  hence thesis by A1;
end;
