Returns the implicit feature associated with the given implicit value.

def implied_feature (implicit_value):
    """ Returns the implicit feature associated with the given implicit value.
    """
    assert isinstance(implicit_value, basestring)
    components = implicit_value.split('-')

    if components[0] not in __implicit_features:
        raise InvalidValue ("'%s' is not a value of an implicit feature" % implicit_value)

    return __implicit_features[components[0]]