Worked on hardening the Fraction arithmetic layer in the kiesraad/abacus repository, focusing on improving correctness and stability for financial calculations. Addressed a critical bug by enforcing preconditions in Rust to prevent construction of Fractions with zero denominators and to ensure division operations do not proceed with a zero-numerator divisor. Updated unit tests to expect panics on invalid inputs, thereby catching errors early in the development cycle. This approach enhanced backend reliability by reducing the risk of runtime panics in production math paths. Emphasized robust error handling and safety practices, leveraging Rust’s type system and unit testing capabilities throughout the process.