is_simple and OGC SFA compliance

[TreeHunter9]

Hi everyone! I have a question about the OGC SFA compliance of implementation of is_simple function for linestring. As OGC SFA says:

A Curve is simple if it does not pass through the same Point twice with the possible exception of the two end points.

And that's it. So, if I understand correctly, it means that empty linestring is considered simple. But in boost implementation I can see that empty linestring is not considered simple. So, it's my interpretation of standard is wrong or implementation?