'Returns the probability of the given symbol sequence. If the sequence
is labelled, then returns the joint probability of the symbol, state
sequence. Otherwise, uses the forward algorithm to find the
probability over all label sequences.
:return: the probability of the sequence
:rtype: float
:param sequence: the sequence of symbols which must contain the TEXT
property, and optionally the TAG property
:type sequence:  Token'

def probability(self, sequence):

    return (2 ** self.log_probability(self._transform(sequence)))
