> not generally feasible or computable

You'd be surprised. It really depends on how you define the problem and what your goal is. My goal with bidicalc what to find ONE solution. This makes the problem somewhat possible since when there are an infinity of solution, the goal is just to converge to one. For example solving 100 = X + Y with both X and Y unknown sounds impossible in general, but finding one solution is not so difficult. The idea is that any further constraint that would help choose between the many solutions should be expressed by the user in the spreadsheet itself, rather than hardcoded in the backwards solver.

> What kind of problems can you solve backwardly?

This is the weakness of the project honestly! I made it because I was obsessed with the idea and wanted it to exist, not because I was driven by any use case. You can load some premade examples in the app, but I haven't found any killer use case for it yet. I'm just glad it exists now. You can enter any arbitrary DAG of formulas, update any value, input or output, and everything will update upstream and downstream from your edit and remain valid. That's just extremely satisfying to me.