anchor.diff:  
   ; (defun remove-seq (trie seq &optional (count 1))
   ;   "### Remove a sequence from trie"
  -;   (unless (find-seq trie seq)
  +;   (unless (obtain-seq trie seq)
   ;     (return-from remove-seq nil))
   ;   (decf (trie-width trie) (max count 1))
   ;   (when (zerop (length seq))
  
positive.code: 
  (defun trie-prob (root suffix)
    "Returns probability of suffix on given trie."
    (let ((node (find-seq root suffix)))
      (when node
        (/ (trie-width node) (trie-width root)))))
  
  
negative.code: 
  ;;; Retrieval
  
  (defun obtain-seq (trie seq)
    "Get a sequence from trie by following the keys in the sequence."
    (when (zerop (length seq))
      (return-from obtain-seq trie))
    (let ((symbol (find-key trie (elt seq 0))))
      (when symbol
        (obtain-seq symbol (subseq seq 1)))))
  
  (defun find-key (trie key)
    "Get a symbol matching key from trie's branches."
    (find key (trie-branches trie) :test #'equal :key #'trie-key))
  
  (defun sym-interval (trie key)
  