def sorensen(seq1, seq2):
	"""Compute the Sorensen distance between the two sequences `seq1` and `seq2`.
	They should contain hashable items.
	
	The return value is a float between 0 and 1, where 0 means equal, and 1 totally different.
	"""
	set1, set2 = set(seq1), set(seq2)
	return 1 - (2 * len(set1 & set2) / float(len(set1) + len(set2)))