Feature Request: Parametrize the default functions for the engine

[sonalys]

Is your feature request related to a problem? Please describe.
I don't want the person writing the rules to have access to all default functions, therefore I would like to have the possibility of either using the default set of functions, or implement my own set of default functions.

Describe the solution you'd like
An option or parameter is accepted by the engine to give a struct containing the default functions