def parse_paragraph_heading_depth(self, markup):
return markup.count("<STR_LIT:=>")/<NUM_LIT:2> - <NUM_LIT:1><EOL>
Returns the depth of a heading.

        The depth determines parent and child relations,
        which headings (and hence which paragraphs) are a child to a heading higher up.
        Returns 0 for <h1> =, 1 for <h2> ==, etc.

        Called from parse_paragraphs() method.