(defun spectra-compare (s1 s2 &optional (eps 1e-2))
  "Compare spectrum multisets S1 and S2 by using convolution and
return its largest multiplicity and corresponding element."
  (let ((diffs (make-hash-table)))
    (dolist (x s1)
      (dolist (y s2)
        (let ((d (truncate (/ (- x y) eps))))
          (incf (gethash d diffs 0)))))
    (loop with best-d = 0
          with best-m = 0
          for d being each hash-key of diffs
          using (hash-value m)
          when (> m best-m)
          do (setf best-d d best-m m)
          finally (return (values best-m (* best-d eps))))))