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@olivierth:
To add to Tomas‘ answer:
The functions that you use and know in vex are all predefined functions (by SideFX devs).
But you can create your own user-defined function if you need one.

The user defined function needs a type (float, vector, void etc) and arguments (as „inputs“), similar to any function you know from VEX already. And will output a result of the defined type.

Also, you can overload functions (the multiple „versions“ some VEX functions have in the documentation) to output a different data type.

In your example above, this exact function you need is not available in VEX, therefore you create your „own version“ of it. Hence making it available for reuse.

Hope this helps

You could dynamically create constraints (attach) once they hit the big sphere

you can also use the windingNumber SOP to check if points are inside a geometry.
since packed geo is just "points" representing transforms of your pieces (plus collision hulls and a few other attributes and properties attached to them for RBD sims), you can then group those points (based on their winding number) and in DOPs simply apply your desired force to that group only.

hope this helps