Returns (hit, index) tuple.
        hit is a boolean, signifying label presence in the cache
        index is an integer, the instruction index for the label entry

def query(self, label):
        """
        Returns (hit, index) tuple.
        hit is a boolean, signifying label presence in the cache
        index is an integer, the instruction index for the label entry
        """
        try:
            return True, self.cache[label]
        except KeyError, e:
            return False, 0