Can an overlay change a function definition?

Suppose I had a yml file that declared a function that is called throughout that yml file. Can I add an overlay that redefines that function to have different content?

No file, test, or entry point is named. Start by locating ytt's overlay and function-definition handling, then determine whether an overlay can redefine a function used elsewhere in the same YAML file. Done means the supported behavior and its scope are documented or implemented with coverage for this case.

Written by the indexing model from the issue text.