def get_heading_text_lines(heading_regions):
    """
    Given a list of TextRegion objects ``heading_regions`` (of type heading) return all of their text lines as one big
    list.
    :param heading_regions: List of TextRegion objects of type heading.
    :type heading_regions: List[TextRegion]
    :return: List of all TextLine objects belonging to the heading regions.
    """
    text_lines = []
    for heading_region in heading_regions:
        text_lines.extend(heading_region.text_lines)
    return text_lines