code: def getNbVariants(self, x1, x2 = -1) :
  		
  		if x2 == -1 :
  			xx2 = len(self.defaultSequence)
  		else :
  			xx2 = x2
  		
  		nbP = 1
  		for p in self.polymorphisms:
  			if x1 <= p[0] and p[0] <= xx2 :
  				nbP *= len(p[1])
  		
  		return nbP
docs: returns the nb of variants of sequences between x1 and x2