def edgeFromNodes(n1, n2):
    """Return the edge coorsponding to nodes n1, n2"""
    if (n1 == 0 and n2 == 1) or (n1 == 1 and n2 == 0):
        return 0
    elif (n1 == 0 and n2 == 2) or (n1 == 2 and n2 == 0):
        return 2
    elif (n1 == 3 and n2 == 1) or (n1 == 1 and n2 == 3):
        return 3
    elif (n1 == 3 and n2 == 2) or (n1 == 2 and n2 == 3):
        return 1