code: def searchitem(self, item, threshold=None):
          
          return self.search(self.key(item), threshold)
docs: Search the index for items whose key exceeds the threshold
          similarity to the key of the given item.
  
          :return: list of pairs of (item, similarity) by decreasing similarity.
  
          >>> from ngram import NGram
          >>> n = NGram([(0, "SPAM"), (1, "SPAN"), (2, "EG"),
          ... (3, "SPANN")], key=lambda x:x[1])
          >>> sorted(n.searchitem((2, "SPA"), 0.35))
          [((0, 'SPAM'), 0.375), ((1, 'SPAN'), 0.375)]