theorem Th7:
  (m1 + m2) - m2 = m1
proof
  let p be object;
  assume
A1: p in P;
  m2 c= (m1 + m2) by Th4;
  hence ((m1 + m2) - m2).p = (m1 + m2) .p - m2.p by A1,Def5
    .= m1.p + m2.p - m2.p by A1,Def4
    .= m1.p;
end;
