I was looking at Tverski contrast distance :
d(x,y) = a * f(x intersect y) - b * f(x-y) - c * f(y-x)
in binary :
d(x,y) = a * count(x&y) - b * count(x&~y) - c * count(~x&y)
It is said that it is SET DISTANCE metric !!
How to understand this ? Beside the obvious, what does it mean to be a SET metric. For distance like Euclidean and Hamming it is easier to visualize it as distance between two points in N-dimentional space, not so for a SET !