tance between
        the last character of the first span and the first character of the second span in
        sequence, so it is always independent of the order of the two span.

        This is negative if the spans overlap.

        Note: this can be called with an Annotation or AnnotationSet instead of `start` and `end`
          (see gatenlp._utils.support_annotation_or_set)

        Args:
          start: start offset of span
          end: end offset of span

        Returns:
          size of gap
