Displaying total logged time of a tree in org-mode

In org-mode, I often log time for tasks with C-c-C-x-C-i (org-clock-in)
and C-c C-x C-o (org-clock-out). At the end of the day, it is nice to be
able to sum up times for all tasks.

    * Top task 1
    ** Sub task 1
       :LOGBOOK:
       CLOCK: [2018-08-29 on. 09:05]--[2018-08-29 on. 11:37] =>  2:32
       :END:
    ** Sub task 2
       :LOGBOOK:
       CLOCK: [2018-08-30 to. 13:14]--[2018-08-30 to. 14:38] =>  1:24
       CLOCK: [2018-08-30 to. 09:46]--[2018-08-30 to. 11:36] =>  1:50
       :END:
    * Top task 2
    ** Sub task 3
       :LOGBOOK:
       CLOCK: [2018-08-30 to. 13:14]--[2018-08-30 to. 14:38] =>  1:24
       CLOCK: [2018-08-30 to. 09:46]--[2018-08-30 to. 11:36] =>  1:50
       :END:
    ** Sub task 4
       :LOGBOOK:
       CLOCK: [2018-08-30 to. 21:02]--[2018-08-30 to. 21:02] =>  0:00
       CLOCK: [2018-08-30 to. 14:38]--[2018-08-30 to. 15:08] =>  0:30
       :END:

Luckily, org-mode contains a short cut for this: C-c C-x C-d. The
command displays the sum of all times:

    * Top task 1***********************************************        5:46
    ** Sub task 1**********************************************        2:32
       :LOGBOOK:
       CLOCK: [2018-08-29 on. 09:05]--[2018-08-29 on. 11:37] =>  2:32
       :END:
    ** Sub task 2**********************************************        3:14
       :LOGBOOK:
       CLOCK: [2018-08-30 to. 13:14]--[2018-08-30 to. 14:38] =>  1:24
       CLOCK: [2018-08-30 to. 09:46]--[2018-08-30 to. 11:36] =>  1:50
       :END:
    * Top task 2***********************************************        3:44
    ** Sub task 3**********************************************        3:14
       :LOGBOOK:
       CLOCK: [2018-08-30 to. 13:14]--[2018-08-30 to. 14:38] =>  1:24
       CLOCK: [2018-08-30 to. 09:46]--[2018-08-30 to. 11:36] =>  1:50
       :END:
    ** Sub task 4**********************************************        0:30
       :LOGBOOK:
       CLOCK: [2018-08-30 to. 21:02]--[2018-08-30 to. 21:02] =>  0:00
       CLOCK: [2018-08-30 to. 14:38]--[2018-08-30 to. 15:08] =>  0:30
       :END:

Another useful command is C-c C-c which updates a LOGBOOK row after you
manually edited it.