Alastair Reid enhanced the xdslproject/xdsl repository by extending its Arithmetic Interpreter to support signed integer operations, including shift left, shift right, division, remainder, and floor division. He implemented these features in Python, focusing on robust interpreter development and comprehensive unit testing. By validating behavior across both positive and negative inputs, Alastair ensured the interpreter’s correctness and reliability for a wider range of arithmetic scenarios. His work improved the depth of numeric capabilities within the domain-specific language, laying a solid foundation for future optimizations and preventing regressions through increased test coverage. The engineering approach emphasized maintainability and thorough validation.