[Feature]: Template Expression with user defined expressions as variables

[jiangr100]

Feature Request

Hello Miles,

Is it currently possible to implement the following functionality: I want to define a variable x = p1 * a + p2 * b + p3 * c, where a, b, and c are from the input data, and p1 to p3 are their parameters. When I use the variable x multiple times in an equation, I want each instance to have a different set of parameters. I know we can currently use parametrized expressions, but this creates only one set of parameters (so different instances of x in one expression use the same parameters).

Also, I've realized that when running like this, the complexity measurement can be different. The variable x is considered to have a complexity of 1, even though it is itself a combined expression. This is good for my application, but I'm curious how this is achieved and if we can have a more flexible approach for controlling complexity. I would like to implement some custom mutations.

Thanks, and please let me know if anything is unclear.