[Feature Request] Function Definitions

[iamrecursion]

Is your feature request related to a problem? Please describe.
Sometimes I've found that I want to evaluate a given mathematical expression for a series of inputs. Right now, I end up writing out the expression with substitutions as necessary, but it would be great to be able to define an ad-hoc function that is then used.

Describe the solution you'd like
A way to define a function on a line such as def_solve(signature, expression) that can then be used on subsequent lines. The signature would be something like my_fn(a, b) and then I could use a and b in the expression.

Describe alternatives you've considered
Writing a provider, but this is very heavyweight for only locally-reusable things.

[iamrecursion]

This would have allowed me to define my own version of #31!