A difficult prerequisite for that might be untangling a very unatomic codebase into testable chunks. And to determine a feasible "level of abstraction" to write

A difficult prerequisite for that might be untangling a very unatomic codebase into testable chunks. And to determine a feasible "level of abstraction" to write tests for. Testing a full pipeline of a numerical library might be as impractical as testing super tiny functions, because both won't allow you to really work on the codebase.