Answer by Daniel Lubarov for Golfing: How many unit-length squares in a list of 2d coordinates?

Python, 61 chars

f=lambda l:sum(1for x,y in l if{(x+1,y),(x,y+1),(x+1,y+1)}<l)

Sample:

>>> f({(0,0), (0,1), (1,1), (1,0), (0,2), (1,2)})2>>> f({(0,0), (0,1), (1,1), (1,0), (0,2), (1,2), (2,2), (2,1)})3>>> f({(0,0), (0,1), (1,1), (1,0), (0,2), (1,2), (2,2), (2,1), (2,0)})4>>> f({(0,0), (0,1), (1,1), (1,0), (0,2), (1,2), (2,2), (2,1), (2,0), (9,9)})4