Replace a local include statement with the file content. Necessary
        adjustments are applied to the content: cut between certain headings,
        strip the top heading, set heading level.

        :param file_path: Path to the included file
        :param from_heading: Include starting from this heading
        :param to_heading: Include up to this heading (not including the heading itself)
        :param options: ``sethead``, ``nohead``

        :returns: Included file content

def _process_include(
            self,
            file_path: Path,
            from_heading: str or None = None,
            to_heading: str or None = None,
            options={}
        ) -> str:
        '''Replace a local include statement with the file content. Necessary
        adjustments are applied to the content: cut between certain headings,
        strip the top heading, set heading level.

        :param file_path: Path to the included file
        :param from_heading: Include starting from this heading
        :param to_heading: Include up to this heading (not including the heading itself)
        :param options: ``sethead``, ``nohead``

        :returns: Included file content
        '''

        self.logger.debug(
            f'Included file path: {file_path}, from heading: {from_heading}, ' +
            f'to heading: {to_heading}, options: {options}'
        )

        if file_path.name.startswith('^'):
            file_path = self._find_file(file_path.name[1:], file_path.parent)

        with open(file_path, encoding='utf8') as incl_file:
            incl_content = incl_file.read()

            if from_heading:
                incl_content = self._cut_from_heading_to_heading(
                    incl_content,
                    from_heading,
                    to_heading,
                    options
                )

            else:
                incl_content = self._cut_to_heading(
                    incl_content,
                    to_heading,
                    options
                )

            incl_content = self._adjust_image_paths(incl_content, file_path)

        return incl_content