def cradmin_page_cover_bem_block(context):
    """
    Get the page-cover BEM block.

    Tries to find the BEM block by lookup in this order:

    - If we have the `page_cover_bem_block` variable in the template context, use that.
    - Try to use request.cradmin_app.page_cover_bem_block.
    - Try to use request.cradmin_instance.page_cover_bem_block.
    - Fall back on `"adminui-page-cover"`.
    """
    request = context['request']
    context_bem_block = context.get('page_cover_bem_block', None)
    if context_bem_block:
        return context_bem_block
    cradmin_app = getattr(request, 'cradmin_app', None)
    if cradmin_app:
        return cradmin_app.page_cover_bem_block
    cradmin_instance = getattr(request, 'cradmin_instance', None)
    if cradmin_instance:
        return cradmin_instance.page_cover_bem_block
    return 'adminui-page-cover'