theorem Th3:
  B1 c= B2 implies B1+p c= B2+p
proof
  assume
A1: B1 c= B2;
  let p1 be object;
  assume p1 in B1+p;
  then ex p2 being Point of T st p1 =p2+p & p2 in B1;
  hence thesis by A1;
end;
