[...] The two options on the table are:

1) Allow references to any value that has been provided in any way
2) Allow references only to parameters to the left

Option 2 is a simple SyntaxError on compilation (you won't even get as
far as the def statement). Option 1 allows everything all up to the
point where you call it, but then might raise UnboundLocalError if you
refer to something that wasn't passed.