theorem
  (F/\G) -- r = (F--r) /\ (G--r)
proof
  thus (F/\G) -- r = --(r--(F/\G)) by Th60
    .= --((r--F) /\ (r--G)) by Th157
    .= (--(r--F)) /\ (--(r--G)) by Th6
    .= (--(r--F)) /\ (G--r) by Th60
    .= (F--r) /\ (G--r) by Th60;
end;
