Pioneers whose defining contribution is the design of a programming language, the discipline of building software that is reliable and maintainable, verifying that software behaves correctly, or the software layer that manages a computer's hardware on the programs' behalf, the whole span the ACM Computing Classification System groups as Software and Its Engineering. This folds together four earlier pioneer fields: Programming Language Theory (John Backus's Fortran, Guido van Rossum's Python, James Gosling's Java, Bjarne Stroustrup's C++), Software Engineering (Margaret Hamilton's Apollo Guidance Computer flight software, Fred Brooks's System/360 management, Barry Boehm's COCOMO model, Richard Stallman's free software movement), Formal Methods (Robin Milner's LCF prover, Amir Pnueli's temporal logic, Edmund M. Clarke and E. Allen Emerson's model checking) and Operating Systems (Dennis Ritchie and Ken Thompson's Unix, Linus Torvalds's Linux kernel, Andrew S. Tanenbaum's MINIX). Does not belong here: a pioneer whose defining work is the physical machine this software runs on, filed under Computer Systems Organization, or a foundational question about what computation itself can do rather than how to build it, filed under Theory of Computation.