def pl_fc_entails(KB, q):
    """Use forward chaining to see if a PropDefiniteKB entails symbol q.
    [Fig. 7.15]
    >>> pl_fc_entails(Fig[7,15], expr('Q'))
    True
    """
    count = dict([(c, len(conjuncts(c.args[0]))) for c in KB.clauses
                                                 if c.op == '>>'])
    inferred = DefaultDict(False)
    agenda = [s for s in KB.clauses if is_prop_symbol(s.op)]
    while agenda:
        p = agenda.pop()
        if p == q: return True
        if not inferred[p]:
            inferred[p] = True
            for c in KB.clauses_with_premise(p):
                count[c] -= 1
                if count[c] == 0:
                    agenda.append(c.args[1])
    return False
Use forward chaining to see if a PropDefiniteKB entails symbol q.
    [Fig. 7.15]
    >>> pl_fc_entails(Fig[7,15], expr('Q'))
    True