MAYBE
* Step 1: Failure MAYBE
  + Considered Problem:
      - Strict TRS:
          append(@l1,@l2) -> append#1(@l1,@l2)
          append#1(::(@x,@xs),@l2) -> ::(@x,append(@xs,@l2))
          append#1(nil(),@l2) -> @l2
          subtrees(@t) -> subtrees#1(@t)
          subtrees#1(leaf()) -> nil()
          subtrees#1(node(@x,@t1,@t2)) -> subtrees#2(subtrees(@t1),@t1,@t2,@x)
          subtrees#2(@l1,@t1,@t2,@x) -> subtrees#3(subtrees(@t2),@l1,@t1,@t2,@x)
          subtrees#3(@l2,@l1,@t1,@t2,@x) -> ::(node(@x,@t1,@t2),append(@l1,@l2))
      - Signature:
          {append/2,append#1/2,subtrees/1,subtrees#1/1,subtrees#2/4,subtrees#3/5} / {::/2,leaf/0,nil/0,node/3}
      - Obligation:
          innermost runtime complexity wrt. defined symbols {append,append#1,subtrees,subtrees#1,subtrees#2
          ,subtrees#3} and constructors {::,leaf,nil,node}
  + Applied Processor:
      NaturalPI {shape = Mixed 3, restrict = NoRestrict, uargs = UArgs, urules = URules, selector = Nothing}
  + Details:
      Incompatible
MAYBE