theorem Th159:
  r -- (F\+\G) = (r--F) \+\ (r--G)
proof
  thus r -- (F\+\G) = r ++ ((--F)\+\(--G)) by Th8
    .= (r++--F) \+\ (r++--G) by Th140
    .= (r--F) \+\ (r--G);
end;
