It would be nice if similarity is commutative.

Requirements:

diff("", "") = 0
diff("", "a") > diff("", """)
diff("one two three", "one two four") > diff("one two three", "one two three")
diff("one two three", "one five four") > diff("one two three", "one three three")
diff("one two three", "one five four") > diff("one two three", "one three four")