theorem
  B (+) (X/\Y) c= (B (+) X)/\(B (+) Y)
proof
  B (+) (X/\Y) = (X/\Y) (+) B by Th12;
  then B (+) (X/\Y) c= (X (+) B)/\(Y (+) B) by Th34;
  then B (+) (X/\Y) c= (B (+) X)/\(Y (+) B) by Th12;
  hence thesis by Th12;
end;
