RemoveGroup POP context not working?

[Alkyum]

Hi guys,

I'm trying to do a simple thing that it's easy to do with Thinking Particle. Switch a particle from a group to an other.
Maybe I'm not doing it well so I'm asking you.

I create a sphere with particle on it, particle on collision ("drop" group) are replicating "trails",

but I want that when particles are no more in collision to be remove from drop group to not replicate anymore.

I tried to use the removegroup() vex function in a popwrangle but it's not working. So I tried to use it un a SopSolver, same issue.

"Call to undefined function". Did I do something wrong?

Thanks guys

Here a test file to show my problem.

test_removegroup.hipnc