def read_edges(filename):
    with open(filename) as lines:
        for line in lines:
            line = line.strip()
            if line:
                (parent, child) = tuple((x.strip() for x in line.split(',')))
                (yield (FIRST_ITERATION, (parent, {child})))
def read_edges(filename):
    with open(filename) as lines:
        for line in lines:
            line = line.strip()
            if line:
                (parent, child) = tuple((x.strip() for x in line.split(',')))
                (yield (FIRST_ITERATION, (parent, {child})))