If a `swear` ever fails, **the function is deleted from the ...

If a swear ever fails, the function is deleted from the source code. This guarantees that only functionally correct remains in the codebase.

But isn’t a language that deletes code crazy?

No, wanting to keep code that demonstrably has bugs according to its own specifications is crazy. What good could it possibly serve? It is corrupted and must be cleansed from your codebase.

The Vigil programming language, like Python with a contract syntax for pre/post conditions.