The version of the runtime, which a compiled program (a .uc file) records and which has to match the version running it: its MCode is only readable by the runtime that wrote it. ucm (compile, run.compiled) and unisonrt both use this one, so two programs built in the same tree agree. It is made the way ucm makes its own version (unison-cli-mainunisonVersion.hs), from git when this module is compiled.