Return true iff f is an ordinary subfeature of the parent_property's
        feature, or if f is a subfeature of the parent_property's feature
        specific to the parent_property's value.

def is_subfeature_of (parent_property, f):
    """ Return true iff f is an ordinary subfeature of the parent_property's
        feature, or if f is a subfeature of the parent_property's feature
        specific to the parent_property's value.
    """
    if __debug__:
        from .property import Property
        assert isinstance(parent_property, Property)
        assert isinstance(f, Feature)

    if not f.subfeature:
        return False

    p = f.parent
    if not p:
        return False

    parent_feature = p[0]
    parent_value = p[1]

    if parent_feature != parent_property.feature:
        return False

    if parent_value and parent_value != parent_property.value:
        return False

    return True