(defun twos-complement (number bits)
  (declare (integer number)
           (positive-fixnum bits))
  (let ((machine-size (expt 2 bits))
        (max-number-size (expt 2 (- bits 1))))
    (assert (< (- max-number-size) number (1- max-number-size)))
    ;; Using floor instead of mod due to silly sbcl code deletion warning.
    ;; SBCL really ought not be warning on (mod number bits) but it does.
    (nth-value 1
               (if (< number 0)
                   (floor number machine-size)
                   (floor number (- machine-size))))))