https://github.com/pantsbuild/pants/issues/8343#issuecomment-559317489 <= i happened upon

formattable_python_target.py

and believe i have a solution which directly removes boilerplate, in https://github.com/pantsbuild/pants/pull/8542, by allowing an

@union

class to be used directly as an

@rule

param (which produces a copy of the rule for each member of the union)