function handler_append () {
    local THING=${1:?"missing thing parameter"}
    RESULT+=$THING
}

{
    local RESULT

    handler_append 0
    mbfl_location_enter
    {
        mbfl_location_handler "handler_append 1"
        mbfl_location_enter
        {
            mbfl_location_handler "handler_append 2"
            mbfl_location_enter
            {
                mbfl_location_handler "handler_append 3"
            }
            mbfl_location_leave
            mbfl_location_handler "handler_append 4"
        }
        mbfl_location_leave
        mbfl_location_handler "handler_append 5"
    }
    mbfl_location_leave
    handler_append 6

    "$RESULT"   ⇒ 0342516
}