def _list_tool_categories(tl):
    """
    Given a list of dicts `tl` as returned by the `parse_tool_list` method and
    where each list element holds a key `tool_panel_section_id`, return a list
    of unique section IDs.
    """
    category_list = []
    for t in tl:
        category_list.append(t.get('tool_panel_section_id'))
    return set(category_list)