theorem Th234:
  (A++B)///a = A///a ++ B///a
proof
  thus (A++B)///a = a"**(A++B) by Th37
    .= a"**A++a"**B by Th208
    .= A///a++a"**B by Th37
    .= A///a ++ B///a by Th37;
end;
