1026.lean:7:14-7:19: warning: declaration uses 'sorry'
1026.lean:11:2-11:7: warning: declaration uses 'sorry'
foo._unfold : ∀ (n : Nat),
  foo n =
    if n = 0 then 0
    else
      let x := n - 1;
      let_fun this := (_ : True);
      foo x