Builds the implication graph from the formula

def build_graph(formula):
    ''' Builds the implication graph from the formula '''
    graph = {}

    for clause in formula:
        for (lit, _) in clause:
            for neg in [False, True]:
                graph[(lit, neg)] = []

    for ((a_lit, a_neg), (b_lit, b_neg)) in formula:
        add_edge(graph, (a_lit, a_neg), (b_lit, not b_neg))
        add_edge(graph, (b_lit, b_neg), (a_lit, not a_neg))

    return graph