help in algorithm

I have  a self organizing net which aim is clustering words.
Let's think the clustering is about their 2-grams set.
Words then are instances of this class.

class clusterable(str):
  def __abs__(self):# the set of q-grams (to be calculated only once)
    return set([(self+self[0])[n:n+2] for n in range(len(self))])
  def __sub__(self,other): # the q-grams distance between 2 words
    set1=abs(self)
    set2=abs(other)
    return len(set1|set2)-len(set1&set2)

I'm looking for the medium of a set of words, as the word which minimizes the sum of the distances from those words.

Aka:sum([medium-word for word in words])


Thanks for ideas, Paolino







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