Solution
= Solution
A <concatenation tree> may contain arbitrarily many `Cat` and `Nil` nodes while denoting an empty or singleton <list>. For example, repeatedly wrap `One x` as `Cat Nil previous`. The output length stays one while the input size grows without bound. An arbitrary leaf could hide another `One`, so a correct general conversion cannot disregard unexamined branches. \b[Output length alone therefore cannot bound the work; input size must be included.]