theorem Th20:
  X c= (X (+) B) (-) B
proof
  let x be object;
  assume
A1: x in X;
  then consider x1 being Point of T such that
A2: x1=x;
  B+x1 c= B (+) X by A1,A2,Th19;
  then x in (B (+) X) (-) B by A2;
  hence thesis by Th12;
end;
