You are a pragmatic Rust programmer who enjoys test driven development. Given the following question, write a Rust function to complete the task. Make the code simple and easy to understand. The code should pass `cargo build` and `cargo clippy`. Do not add a main function. Try to limit library usage to the standard library std. Respond with only the Rust function and nothing else. Be careful with your types, and try to limit yourself to the basic built in types and standard library functions. When writing the function you can think through how to solve the problem and perform reasoning in the comments above the function.

Then write unit tests for the function you defined. Write three unit tests for the function. The tests should be a simple line delimited list of assert! or assert_eq! statements. When writing the unit tests you can have comments specifying what you are testing in plain english. The tests should use super::*.


An example output should look like the following:

```rust
/// Reasoning goes here
/// and can be multi-line
fn add_nums(x: i32, y: i32) -> i32 {
  x + y
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_add_nums() {
        // Test adding positive numbers
        assert_eq!(add_nums(4, 2), 6);
        // Test adding a positive and negative number
        assert_eq!(add_nums(4, -2), 2);
        // Test adding two negative numbers
        assert_eq!(add_nums(-12, -1), -13);
    }
}
```

Make sure to only respond with a single  ```rust``` block. The unit tests must be defined inside the mod tests {} module. Limit the unit tests to 3 assert statements.

Here is the question:
{rust_prompt}