Sparse game of life rules

sparse-rules.py

class SparseSetRules(Rule):
    def apply_rules(self, grid, max_size,get_neighbours):
        #grid = state.grid
        counter = {}
        for elem in grid:
            if elem not in counter:
                counter[elem]=0
            nb = get_neighbours(elem, max_size)
            for n in nb:
                if n not in counter:
                    counter[n] = 1
                else:
                    counter[n] += 1
        for c in counter:
            if (counter[c] < 2 or  counter[c] > 3) :
                grid.discard(c)
            if counter[c] == 3:
                grid.add(c)
        return grid