[O] How to compute the number of headlines under the given headline?

[Marcin Borkowski]

Hello,

I'd like to count the number of children (alternatively, number of all
descendants) of an Org headline.  Is there a ready-made function for
that?  If not, would using org-forward-heading-same-level and similar
functions in Lisp code be a good idea?

TIA,

-- 
Marcin Borkowski
http://octd.wmi.amu.edu.pl/en/Marcin_Borkowski
Faculty of Mathematics and Computer Science
Adam Mickiewicz University