Trying all simple changes (first order mutations) to executed C, C++ and CUDA source code shows software engineering artefacts are more robust than is often assumed. Of those that compile, up to 89% run without error. Indeed a few mutants are improvements. Program fitness landscapes are smoother. Analysis of these programs, a parallel nVidia GPGPU kernel, all CUDA samples and the GNU C library shows many lines of code and integer values are repeated and may follow Zipf's law.