theorem Th16:
  (m1 + m2) - m1 = m2
proof
A1: m1 c= m1 + m2 by Th4;
  let p be object;
  assume
A2: p in P;
  then ((m1 + m2) - m1).p = (m1 + m2).p - m1.p by A1,Def5
    .= m1.p + m2.p - m1.p by A2,Def4
    .= m2.p;
  hence thesis;
end;
