theorem Th157:
  r -- (F/\G) = (r--F) /\ (r--G)
proof
  thus r -- (F/\G) = r ++ ((--F)/\(--G)) by Th6
    .= (r++--F) /\ (r++--G) by Th137
    .= (r--F) /\ (r--G);
end;
