Returns true iff 'value_string' is a value_string
    of an implicit feature.

def is_implicit_value (value_string):
    """ Returns true iff 'value_string' is a value_string
    of an implicit feature.
    """
    assert isinstance(value_string, basestring)
    if value_string in __implicit_features:
        return __implicit_features[value_string]

    v = value_string.split('-')

    if v[0] not in __implicit_features:
        return False

    feature = __implicit_features[v[0]]

    for subvalue in (v[1:]):
        if not __find_implied_subfeature(feature, subvalue, v[0]):
            return False

    return True