A logic heavy function for automagically creating a breadcrumb.

    It uses the dashboard, panel group(if it exists), then current panel.
    Can also use a "custom_breadcrumb" context item to add extra items.

def breadcrumb_nav(context):
    """A logic heavy function for automagically creating a breadcrumb.

    It uses the dashboard, panel group(if it exists), then current panel.
    Can also use a "custom_breadcrumb" context item to add extra items.
    """
    breadcrumb = []
    dashboard = context.request.horizon['dashboard']
    try:
        panel_groups = dashboard.get_panel_groups()
    except KeyError:
        panel_groups = None
    panel_group = None
    panel = context.request.horizon['panel']

    # Add panel group, if there is one
    if panel_groups:
        for group in panel_groups.values():
            if panel.slug in group.panels and group.slug != 'default':
                panel_group = group
                break

    # Remove panel reference if that is the current page
    if panel.get_absolute_url() == context.request.path:
        panel = None

    # Get custom breadcrumb, if there is one.
    custom_breadcrumb = context.get('custom_breadcrumb')

    # Build list of tuples (name, optional url)
    breadcrumb.append((dashboard.name, None))
    if panel_group:
        breadcrumb.append((panel_group.name, None))
    if panel:
        breadcrumb.append((panel.name, panel.get_absolute_url()))
    if custom_breadcrumb:
        breadcrumb.extend(custom_breadcrumb)
    breadcrumb.append((context.get('page_title'), None))

    return {'breadcrumb': breadcrumb}