def get_literals(cnf):
    """Extract the literals from a cnf formula

    Parameters
    ----------
    cnf :list[set[(string,bool)]]
        The cnf from wich we want to extract the literla

    Returns
    -----
    set[str]
        set of the literals in the cnf
    """
    literals = set()
    for conj in cnf:
        for disj in conj:
            literals.add(disj[0])
    return literals