YES

TRS:

  flatten(nil()) -> nil()
  flatten(unit(x)) -> flatten(x)
  flatten(++(x,y)) -> ++(flatten(x),flatten(y))
  flatten(++(unit(x),y)) -> ++(flatten(x),flatten(y))
  flatten(flatten(x)) -> flatten(x)
  rev(nil()) -> nil()
  rev(unit(x)) -> unit(x)
  rev(++(x,y)) -> ++(rev(y),rev(x))
  rev(rev(x)) -> x
  ++(x,nil()) -> x
  ++(nil(),y) -> y
  ++(++(x,y),z) -> ++(x,++(y,z))

linear polynomial interpretations on N:

  flatten_A(x1) = x1
  flatten#_A(x1) = x1
  nil_A = 1
  nil#_A = 1
  unit_A(x1) = x1 + 1
  unit#_A(x1) = x1 + 1
  ++_A(x1,x2) = x1 + x2 + 1
  ++#_A(x1,x2) = x1 + x2 + 1
  rev_A(x1) = x1
  rev#_A(x1) = x1

precedence:

  flatten = nil = ++ = rev > unit