Alastair Reid extended the Arithmetic Interpreter in the xdslproject/xdsl repository by implementing support for signed integer operations, including shift left, shift right, division, remainder, and floor division. Using Python, he developed these features in arith.py and ensured their correctness through comprehensive unit testing, covering both positive and negative input scenarios. His work focused on expanding the interpreter’s numeric capabilities and reliability, laying groundwork for future optimizations. By enhancing test coverage and validating edge cases, Alastair improved the robustness of arithmetic operations within the interpreter, demonstrating depth in interpreter development, arithmetic operations, and systematic unit testing practices.