Field source for sub fields

[sebakerckhof]

Currently it's not possible to select a field src (e.g. function) on a field that has a parent field.

I assume this has a certain reason, but it's not obvious to me why this wouldn't be possible?

[ukrbublik]

It was done for simplicity.
Rule-groups are complex by its own so adding functions support to its children would require more complicated code.
But it's a good feature request, I can add such support in the future