1673.lean:1:4-1:9: error: fail to show termination for
  foo.a
with errors
failed to infer structural recursion:
Not considering parameter n of foo.a:
  it is unchanged in the recursive calls
Not considering parameter nope of foo.a:
  it is unchanged in the recursive calls
no parameters suitable for structural recursion

well-founded recursion cannot be used, 'foo.a' does not take any (non-fixed) arguments