 at or after start==end is overlapping
        if start == end:
            # in order to check what is overlapping with the zero length span, we need to find
            # intervals which start before the start of that span AND end at least one after that span;
            # or intervals which start at the span and end with the span or after.
            # In other words we look at all intervals which start anywhere before or at the start(=end)
            # of the span.
            # if the start of what 