I am unable to fit that example in this prior description: "if I append to the source file, I can't break previously correct code above the insertion point". Y

I am unable to fit that example in this prior description: "if I append to the source file, I can't break previously correct code above the insertion point". Your foo() example above has code which has been changed, rather than "if I append to the source file". (I re-read my prior comment and realized this may not have been clear.) That a change to code can introduce a bug is something we've all seen. But your prior description is not any kind of change, it was specifically an append. In the context of forward references, I am curious to see something that matched your prior description.