YES(?,O(n^1))

We are left with following problem, upon which TcT provides the
certificate YES(?,O(n^1)).

Strict Trs:
  { flatten(flatten(x)) -> flatten(x)
  , flatten(nil()) -> nil()
  , flatten(unit(x)) -> flatten(x)
  , flatten(++(x, y)) -> ++(flatten(x), flatten(y))
  , flatten(++(unit(x), y)) -> ++(flatten(x), flatten(y))
  , ++(x, nil()) -> x
  , ++(nil(), y) -> y
  , ++(++(x, y), z) -> ++(x, ++(y, z))
  , rev(nil()) -> nil()
  , rev(unit(x)) -> unit(x)
  , rev(++(x, y)) -> ++(rev(y), rev(x))
  , rev(rev(x)) -> x }
Obligation:
  runtime complexity
Answer:
  YES(?,O(n^1))

The problem is match-bounded by 1. The enriched problem is
compatible with the following automaton.
{ flatten_0(2) -> 1
, flatten_1(2) -> 1
, nil_0() -> 1
, nil_0() -> 2
, nil_1() -> 1
, unit_0(2) -> 1
, unit_0(2) -> 2
, unit_1(2) -> 1
, ++_0(2, 2) -> 1
, rev_0(2) -> 1
, 2 -> 1 }

Hurray, we answered YES(?,O(n^1))