Force variables to appear in expression

[zhuyi-bjut]

Hi！ @MilesCranmer
When I do the symbolic regression task, I want multiple variables in the target formula, but the results given by pysr are often fewer variables ( 1 ~ 2 ). How to constrain so that there are enough variables I want in the result formula ( 3 ~ 4 ) ? Although they should not appear from the fitting point of view, they have physical meaning from the physical point of view.
Thanks.

[MilesCranmer]

See #273 (comment) for some relevant discussion.

There's also #44 but that one is much older and a bit out-of-date.