How to define a Hill equation? Or, custom expressions with parameters, how to tell PySR what is a variable and what is a parameter?

[JulianoGianlupi]

Hello all,

Firstly, thank you for this package, I just discovered it and already think it looks awesome. I am wondering though how would one define something like a Hill equation in a way that let's PySR know what is x and what are parameters. The Hill equation is

$$Hill(x) = \frac{x^n}{K^n+x^n}$$

with $$K, n \in \mathbb{R}$$

As all operators in the Hill are very primitive PySR should be able to find the overall expression without any help from custom operators. But my worry is that the overall complexity without a custom operator will be too high and solutions including it would be discarded during optimization.

Thanks!

[MilesCranmer]

Did you look into template expressions at all? Seems useful for this