'Return a probabilistic ``PCFG`` corresponding to the
input string(s).
:param input: a grammar, either in the form of a string or else
as a list of strings.'

@classmethod
def fromstring(cls, input, encoding=None):

    (start, productions) = read_grammar(input, standard_nonterm_parser, probabilistic=True, encoding=encoding)
    return PCFG(start, productions)
