make STARTS-WITH-SUBSEQ not cons unnecessarily

                  :datum sequence

                  :expected-type '(and proper-sequence (not (satisfies emptyp))))))))

(defun starts-with-subseq (prefix sequence &rest args &key (return-suffix nil) &allow-other-keys)

(defun starts-with-subseq (prefix sequence &rest args

                           &key

                           (return-suffix nil return-suffix-supplied-p)

                           &allow-other-keys)

  "Test whether the first elements of SEQUENCE are the same (as per TEST) as the elements of PREFIX.

If RETURN-SUFFIX is T the functions returns, as a second value, a

If RETURN-SUFFIX is T the function returns, as a second value, a

displaced array pointing to the sequence after PREFIX."

  (remove-from-plistf args :return-suffix)

  (declare (dynamic-extent args))

  (let ((sequence-length (length sequence))

        (prefix-length (length prefix)))

    (if (<= prefix-length sequence-length)

        (let ((mismatch (apply #'mismatch prefix sequence args)))

        (let ((mismatch (apply #'mismatch prefix sequence

                               (if return-suffix-supplied-p

                                   (remove-from-plist args :return-suffix)

                                   args))))

          (if mismatch

              (if (< mismatch prefix-length)

                  (values nil nil)