playing chunk of animation forward and backward

( Verge version 3.3.1 – exporting models from Blender )
I need to playback chunks of an animation, in forward and in backward direction.

to play forward from time 3 to time 4 I do:
var x=app.actions[0];
var x.setLoop( v3d.LoopOnce);
x.reset(); x.time=3; x.getClip().duration=4;

to play backward from time 4 to time 3 I tried:
x.reset(); x.time=4; x.timeScale=-1; x.getClip().duration=3;
x.reset(); x.time=4; x.timeScale=-1; x.getClip().duration=-1;

none of these work, so, whats the right way to play a chunk of an animation backward ?

thanks in advance — Silvano