basic backpropagation on restricted output.

if the output changes and it's a smaller subset of possible output values, then we might need to alter our backpropagation.

Example

```
c = a + b
mod(c, 2)
```

this changes the universe of inputs to a + b

The issue names no files, tests, or entry points. Start by locating the backpropagation implementation and reproduce the c = a + b followed by mod(c, 2) example. Clarify the expected behavior for restricted output values before defining what a complete change would require.

Written by the indexing model from the issue text.