F_LOOP is rather simple.
is type int? yes: is it 0? yes: end loop no: decrement call body no: call f_loop
Note the absence of "or less than" in the check for 0. I guess there should be such a check. It's there in the interret_functions.h version.
I will fix it this evening if nobody get's there first. Around line 2854 in amd64.c, for those interreseted. :)