YES

Problem:
 a__fst(0(),Z) -> nil()
 a__fst(s(X),cons(Y,Z)) -> cons(mark(Y),fst(X,Z))
 a__from(X) -> cons(mark(X),from(s(X)))
 a__add(0(),X) -> mark(X)
 a__add(s(X),Y) -> s(add(X,Y))
 a__len(nil()) -> 0()
 a__len(cons(X,Z)) -> s(len(Z))
 mark(fst(X1,X2)) -> a__fst(mark(X1),mark(X2))
 mark(from(X)) -> a__from(mark(X))
 mark(add(X1,X2)) -> a__add(mark(X1),mark(X2))
 mark(len(X)) -> a__len(mark(X))
 mark(0()) -> 0()
 mark(s(X)) -> s(X)
 mark(nil()) -> nil()
 mark(cons(X1,X2)) -> cons(mark(X1),X2)
 a__fst(X1,X2) -> fst(X1,X2)
 a__from(X) -> from(X)
 a__add(X1,X2) -> add(X1,X2)
 a__len(X) -> len(X)

Proof:
 DP Processor:
  DPs:
   a__fst#(s(X),cons(Y,Z)) -> mark#(Y)
   a__from#(X) -> mark#(X)
   a__add#(0(),X) -> mark#(X)
   mark#(fst(X1,X2)) -> mark#(X2)
   mark#(fst(X1,X2)) -> mark#(X1)
   mark#(fst(X1,X2)) -> a__fst#(mark(X1),mark(X2))
   mark#(from(X)) -> mark#(X)
   mark#(from(X)) -> a__from#(mark(X))
   mark#(add(X1,X2)) -> mark#(X2)
   mark#(add(X1,X2)) -> mark#(X1)
   mark#(add(X1,X2)) -> a__add#(mark(X1),mark(X2))
   mark#(len(X)) -> mark#(X)
   mark#(len(X)) -> a__len#(mark(X))
   mark#(cons(X1,X2)) -> mark#(X1)
  TRS:
   a__fst(0(),Z) -> nil()
   a__fst(s(X),cons(Y,Z)) -> cons(mark(Y),fst(X,Z))
   a__from(X) -> cons(mark(X),from(s(X)))
   a__add(0(),X) -> mark(X)
   a__add(s(X),Y) -> s(add(X,Y))
   a__len(nil()) -> 0()
   a__len(cons(X,Z)) -> s(len(Z))
   mark(fst(X1,X2)) -> a__fst(mark(X1),mark(X2))
   mark(from(X)) -> a__from(mark(X))
   mark(add(X1,X2)) -> a__add(mark(X1),mark(X2))
   mark(len(X)) -> a__len(mark(X))
   mark(0()) -> 0()
   mark(s(X)) -> s(X)
   mark(nil()) -> nil()
   mark(cons(X1,X2)) -> cons(mark(X1),X2)
   a__fst(X1,X2) -> fst(X1,X2)
   a__from(X) -> from(X)
   a__add(X1,X2) -> add(X1,X2)
   a__len(X) -> len(X)
  TDG Processor:
   DPs:
    a__fst#(s(X),cons(Y,Z)) -> mark#(Y)
    a__from#(X) -> mark#(X)
    a__add#(0(),X) -> mark#(X)
    mark#(fst(X1,X2)) -> mark#(X2)
    mark#(fst(X1,X2)) -> mark#(X1)
    mark#(fst(X1,X2)) -> a__fst#(mark(X1),mark(X2))
    mark#(from(X)) -> mark#(X)
    mark#(from(X)) -> a__from#(mark(X))
    mark#(add(X1,X2)) -> mark#(X2)
    mark#(add(X1,X2)) -> mark#(X1)
    mark#(add(X1,X2)) -> a__add#(mark(X1),mark(X2))
    mark#(len(X)) -> mark#(X)
    mark#(len(X)) -> a__len#(mark(X))
    mark#(cons(X1,X2)) -> mark#(X1)
   TRS:
    a__fst(0(),Z) -> nil()
    a__fst(s(X),cons(Y,Z)) -> cons(mark(Y),fst(X,Z))
    a__from(X) -> cons(mark(X),from(s(X)))
    a__add(0(),X) -> mark(X)
    a__add(s(X),Y) -> s(add(X,Y))
    a__len(nil()) -> 0()
    a__len(cons(X,Z)) -> s(len(Z))
    mark(fst(X1,X2)) -> a__fst(mark(X1),mark(X2))
    mark(from(X)) -> a__from(mark(X))
    mark(add(X1,X2)) -> a__add(mark(X1),mark(X2))
    mark(len(X)) -> a__len(mark(X))
    mark(0()) -> 0()
    mark(s(X)) -> s(X)
    mark(nil()) -> nil()
    mark(cons(X1,X2)) -> cons(mark(X1),X2)
    a__fst(X1,X2) -> fst(X1,X2)
    a__from(X) -> from(X)
    a__add(X1,X2) -> add(X1,X2)
    a__len(X) -> len(X)
   graph:
    a__add#(0(),X) -> mark#(X) -> mark#(cons(X1,X2)) -> mark#(X1)
    a__add#(0(),X) -> mark#(X) -> mark#(len(X)) -> a__len#(mark(X))
    a__add#(0(),X) -> mark#(X) -> mark#(len(X)) -> mark#(X)
    a__add#(0(),X) -> mark#(X) ->
    mark#(add(X1,X2)) -> a__add#(mark(X1),mark(X2))
    a__add#(0(),X) -> mark#(X) -> mark#(add(X1,X2)) -> mark#(X1)
    a__add#(0(),X) -> mark#(X) -> mark#(add(X1,X2)) -> mark#(X2)
    a__add#(0(),X) -> mark#(X) -> mark#(from(X)) -> a__from#(mark(X))
    a__add#(0(),X) -> mark#(X) -> mark#(from(X)) -> mark#(X)
    a__add#(0(),X) -> mark#(X) ->
    mark#(fst(X1,X2)) -> a__fst#(mark(X1),mark(X2))
    a__add#(0(),X) -> mark#(X) -> mark#(fst(X1,X2)) -> mark#(X1)
    a__add#(0(),X) -> mark#(X) -> mark#(fst(X1,X2)) -> mark#(X2)
    a__from#(X) -> mark#(X) -> mark#(cons(X1,X2)) -> mark#(X1)
    a__from#(X) -> mark#(X) -> mark#(len(X)) -> a__len#(mark(X))
    a__from#(X) -> mark#(X) -> mark#(len(X)) -> mark#(X)
    a__from#(X) -> mark#(X) ->
    mark#(add(X1,X2)) -> a__add#(mark(X1),mark(X2))
    a__from#(X) -> mark#(X) -> mark#(add(X1,X2)) -> mark#(X1)
    a__from#(X) -> mark#(X) -> mark#(add(X1,X2)) -> mark#(X2)
    a__from#(X) -> mark#(X) -> mark#(from(X)) -> a__from#(mark(X))
    a__from#(X) -> mark#(X) -> mark#(from(X)) -> mark#(X)
    a__from#(X) -> mark#(X) ->
    mark#(fst(X1,X2)) -> a__fst#(mark(X1),mark(X2))
    a__from#(X) -> mark#(X) -> mark#(fst(X1,X2)) -> mark#(X1)
    a__from#(X) -> mark#(X) -> mark#(fst(X1,X2)) -> mark#(X2)
    mark#(len(X)) -> mark#(X) -> mark#(cons(X1,X2)) -> mark#(X1)
    mark#(len(X)) -> mark#(X) -> mark#(len(X)) -> a__len#(mark(X))
    mark#(len(X)) -> mark#(X) -> mark#(len(X)) -> mark#(X)
    mark#(len(X)) -> mark#(X) ->
    mark#(add(X1,X2)) -> a__add#(mark(X1),mark(X2))
    mark#(len(X)) -> mark#(X) -> mark#(add(X1,X2)) -> mark#(X1)
    mark#(len(X)) -> mark#(X) -> mark#(add(X1,X2)) -> mark#(X2)
    mark#(len(X)) -> mark#(X) -> mark#(from(X)) -> a__from#(mark(X))
    mark#(len(X)) -> mark#(X) -> mark#(from(X)) -> mark#(X)
    mark#(len(X)) -> mark#(X) ->
    mark#(fst(X1,X2)) -> a__fst#(mark(X1),mark(X2))
    mark#(len(X)) -> mark#(X) -> mark#(fst(X1,X2)) -> mark#(X1)
    mark#(len(X)) -> mark#(X) ->
    mark#(fst(X1,X2)) -> mark#(X2)
    mark#(add(X1,X2)) -> a__add#(mark(X1),mark(X2)) ->
    a__add#(0(),X) -> mark#(X)
    mark#(add(X1,X2)) -> mark#(X2) -> mark#(cons(X1,X2)) -> mark#(X1)
    mark#(add(X1,X2)) -> mark#(X2) ->
    mark#(len(X)) -> a__len#(mark(X))
    mark#(add(X1,X2)) -> mark#(X2) -> mark#(len(X)) -> mark#(X)
    mark#(add(X1,X2)) -> mark#(X2) ->
    mark#(add(X1,X2)) -> a__add#(mark(X1),mark(X2))
    mark#(add(X1,X2)) -> mark#(X2) -> mark#(add(X1,X2)) -> mark#(X1)
    mark#(add(X1,X2)) -> mark#(X2) -> mark#(add(X1,X2)) -> mark#(X2)
    mark#(add(X1,X2)) -> mark#(X2) ->
    mark#(from(X)) -> a__from#(mark(X))
    mark#(add(X1,X2)) -> mark#(X2) -> mark#(from(X)) -> mark#(X)
    mark#(add(X1,X2)) -> mark#(X2) ->
    mark#(fst(X1,X2)) -> a__fst#(mark(X1),mark(X2))
    mark#(add(X1,X2)) -> mark#(X2) -> mark#(fst(X1,X2)) -> mark#(X1)
    mark#(add(X1,X2)) -> mark#(X2) -> mark#(fst(X1,X2)) -> mark#(X2)
    mark#(add(X1,X2)) -> mark#(X1) -> mark#(cons(X1,X2)) -> mark#(X1)
    mark#(add(X1,X2)) -> mark#(X1) ->
    mark#(len(X)) -> a__len#(mark(X))
    mark#(add(X1,X2)) -> mark#(X1) -> mark#(len(X)) -> mark#(X)
    mark#(add(X1,X2)) -> mark#(X1) ->
    mark#(add(X1,X2)) -> a__add#(mark(X1),mark(X2))
    mark#(add(X1,X2)) -> mark#(X1) -> mark#(add(X1,X2)) -> mark#(X1)
    mark#(add(X1,X2)) -> mark#(X1) -> mark#(add(X1,X2)) -> mark#(X2)
    mark#(add(X1,X2)) -> mark#(X1) ->
    mark#(from(X)) -> a__from#(mark(X))
    mark#(add(X1,X2)) -> mark#(X1) -> mark#(from(X)) -> mark#(X)
    mark#(add(X1,X2)) -> mark#(X1) ->
    mark#(fst(X1,X2)) -> a__fst#(mark(X1),mark(X2))
    mark#(add(X1,X2)) -> mark#(X1) -> mark#(fst(X1,X2)) -> mark#(X1)
    mark#(add(X1,X2)) -> mark#(X1) ->
    mark#(fst(X1,X2)) -> mark#(X2)
    mark#(from(X)) -> a__from#(mark(X)) -> a__from#(X) -> mark#(X)
    mark#(from(X)) -> mark#(X) -> mark#(cons(X1,X2)) -> mark#(X1)
    mark#(from(X)) -> mark#(X) -> mark#(len(X)) -> a__len#(mark(X))
    mark#(from(X)) -> mark#(X) -> mark#(len(X)) -> mark#(X)
    mark#(from(X)) -> mark#(X) ->
    mark#(add(X1,X2)) -> a__add#(mark(X1),mark(X2))
    mark#(from(X)) -> mark#(X) -> mark#(add(X1,X2)) -> mark#(X1)
    mark#(from(X)) -> mark#(X) -> mark#(add(X1,X2)) -> mark#(X2)
    mark#(from(X)) -> mark#(X) -> mark#(from(X)) -> a__from#(mark(X))
    mark#(from(X)) -> mark#(X) -> mark#(from(X)) -> mark#(X)
    mark#(from(X)) -> mark#(X) ->
    mark#(fst(X1,X2)) -> a__fst#(mark(X1),mark(X2))
    mark#(from(X)) -> mark#(X) -> mark#(fst(X1,X2)) -> mark#(X1)
    mark#(from(X)) -> mark#(X) -> mark#(fst(X1,X2)) -> mark#(X2)
    mark#(fst(X1,X2)) -> mark#(X2) -> mark#(cons(X1,X2)) -> mark#(X1)
    mark#(fst(X1,X2)) -> mark#(X2) ->
    mark#(len(X)) -> a__len#(mark(X))
    mark#(fst(X1,X2)) -> mark#(X2) -> mark#(len(X)) -> mark#(X)
    mark#(fst(X1,X2)) -> mark#(X2) ->
    mark#(add(X1,X2)) -> a__add#(mark(X1),mark(X2))
    mark#(fst(X1,X2)) -> mark#(X2) -> mark#(add(X1,X2)) -> mark#(X1)
    mark#(fst(X1,X2)) -> mark#(X2) -> mark#(add(X1,X2)) -> mark#(X2)
    mark#(fst(X1,X2)) -> mark#(X2) ->
    mark#(from(X)) -> a__from#(mark(X))
    mark#(fst(X1,X2)) -> mark#(X2) -> mark#(from(X)) -> mark#(X)
    mark#(fst(X1,X2)) -> mark#(X2) ->
    mark#(fst(X1,X2)) -> a__fst#(mark(X1),mark(X2))
    mark#(fst(X1,X2)) -> mark#(X2) -> mark#(fst(X1,X2)) -> mark#(X1)
    mark#(fst(X1,X2)) -> mark#(X2) -> mark#(fst(X1,X2)) -> mark#(X2)
    mark#(fst(X1,X2)) -> mark#(X1) -> mark#(cons(X1,X2)) -> mark#(X1)
    mark#(fst(X1,X2)) -> mark#(X1) ->
    mark#(len(X)) -> a__len#(mark(X))
    mark#(fst(X1,X2)) -> mark#(X1) -> mark#(len(X)) -> mark#(X)
    mark#(fst(X1,X2)) -> mark#(X1) ->
    mark#(add(X1,X2)) -> a__add#(mark(X1),mark(X2))
    mark#(fst(X1,X2)) -> mark#(X1) -> mark#(add(X1,X2)) -> mark#(X1)
    mark#(fst(X1,X2)) -> mark#(X1) -> mark#(add(X1,X2)) -> mark#(X2)
    mark#(fst(X1,X2)) -> mark#(X1) ->
    mark#(from(X)) -> a__from#(mark(X))
    mark#(fst(X1,X2)) -> mark#(X1) -> mark#(from(X)) -> mark#(X)
    mark#(fst(X1,X2)) -> mark#(X1) ->
    mark#(fst(X1,X2)) -> a__fst#(mark(X1),mark(X2))
    mark#(fst(X1,X2)) -> mark#(X1) -> mark#(fst(X1,X2)) -> mark#(X1)
    mark#(fst(X1,X2)) -> mark#(X1) ->
    mark#(fst(X1,X2)) -> mark#(X2)
    mark#(fst(X1,X2)) -> a__fst#(mark(X1),mark(X2)) ->
    a__fst#(s(X),cons(Y,Z)) -> mark#(Y)
    mark#(cons(X1,X2)) -> mark#(X1) ->
    mark#(cons(X1,X2)) -> mark#(X1)
    mark#(cons(X1,X2)) -> mark#(X1) ->
    mark#(len(X)) -> a__len#(mark(X))
    mark#(cons(X1,X2)) -> mark#(X1) -> mark#(len(X)) -> mark#(X)
    mark#(cons(X1,X2)) -> mark#(X1) ->
    mark#(add(X1,X2)) -> a__add#(mark(X1),mark(X2))
    mark#(cons(X1,X2)) -> mark#(X1) ->
    mark#(add(X1,X2)) -> mark#(X1)
    mark#(cons(X1,X2)) -> mark#(X1) ->
    mark#(add(X1,X2)) -> mark#(X2)
    mark#(cons(X1,X2)) -> mark#(X1) ->
    mark#(from(X)) -> a__from#(mark(X))
    mark#(cons(X1,X2)) -> mark#(X1) -> mark#(from(X)) -> mark#(X)
    mark#(cons(X1,X2)) -> mark#(X1) ->
    mark#(fst(X1,X2)) -> a__fst#(mark(X1),mark(X2))
    mark#(cons(X1,X2)) -> mark#(X1) ->
    mark#(fst(X1,X2)) -> mark#(X1)
    mark#(cons(X1,X2)) -> mark#(X1) ->
    mark#(fst(X1,X2)) -> mark#(X2)
    a__fst#(s(X),cons(Y,Z)) -> mark#(Y) ->
    mark#(cons(X1,X2)) -> mark#(X1)
    a__fst#(s(X),cons(Y,Z)) -> mark#(Y) ->
    mark#(len(X)) -> a__len#(mark(X))
    a__fst#(s(X),cons(Y,Z)) -> mark#(Y) ->
    mark#(len(X)) -> mark#(X)
    a__fst#(s(X),cons(Y,Z)) -> mark#(Y) ->
    mark#(add(X1,X2)) -> a__add#(mark(X1),mark(X2))
    a__fst#(s(X),cons(Y,Z)) -> mark#(Y) ->
    mark#(add(X1,X2)) -> mark#(X1)
    a__fst#(s(X),cons(Y,Z)) -> mark#(Y) ->
    mark#(add(X1,X2)) -> mark#(X2)
    a__fst#(s(X),cons(Y,Z)) -> mark#(Y) ->
    mark#(from(X)) -> a__from#(mark(X))
    a__fst#(s(X),cons(Y,Z)) -> mark#(Y) ->
    mark#(from(X)) -> mark#(X)
    a__fst#(s(X),cons(Y,Z)) -> mark#(Y) ->
    mark#(fst(X1,X2)) -> a__fst#(mark(X1),mark(X2))
    a__fst#(s(X),cons(Y,Z)) -> mark#(Y) ->
    mark#(fst(X1,X2)) -> mark#(X1)
    a__fst#(s(X),cons(Y,Z)) -> mark#(Y) -> mark#(fst(X1,X2)) -> mark#(X2)
   SCC Processor:
    #sccs: 1
    #rules: 13
    #arcs: 113/196
    DPs:
     a__add#(0(),X) -> mark#(X)
     mark#(fst(X1,X2)) -> mark#(X2)
     mark#(fst(X1,X2)) -> mark#(X1)
     mark#(fst(X1,X2)) -> a__fst#(mark(X1),mark(X2))
     a__fst#(s(X),cons(Y,Z)) -> mark#(Y)
     mark#(from(X)) -> mark#(X)
     mark#(from(X)) -> a__from#(mark(X))
     a__from#(X) -> mark#(X)
     mark#(add(X1,X2)) -> mark#(X2)
     mark#(add(X1,X2)) -> mark#(X1)
     mark#(add(X1,X2)) -> a__add#(mark(X1),mark(X2))
     mark#(len(X)) -> mark#(X)
     mark#(cons(X1,X2)) -> mark#(X1)
    TRS:
     a__fst(0(),Z) -> nil()
     a__fst(s(X),cons(Y,Z)) -> cons(mark(Y),fst(X,Z))
     a__from(X) -> cons(mark(X),from(s(X)))
     a__add(0(),X) -> mark(X)
     a__add(s(X),Y) -> s(add(X,Y))
     a__len(nil()) -> 0()
     a__len(cons(X,Z)) -> s(len(Z))
     mark(fst(X1,X2)) -> a__fst(mark(X1),mark(X2))
     mark(from(X)) -> a__from(mark(X))
     mark(add(X1,X2)) -> a__add(mark(X1),mark(X2))
     mark(len(X)) -> a__len(mark(X))
     mark(0()) -> 0()
     mark(s(X)) -> s(X)
     mark(nil()) -> nil()
     mark(cons(X1,X2)) -> cons(mark(X1),X2)
     a__fst(X1,X2) -> fst(X1,X2)
     a__from(X) -> from(X)
     a__add(X1,X2) -> add(X1,X2)
     a__len(X) -> len(X)
    Matrix Interpretation Processor: dim=1
     
     interpretation:
      [a__add#](x0, x1) = x1 + 1,
      
      [a__from#](x0) = 2x0 + 4,
      
      [mark#](x0) = x0,
      
      [a__fst#](x0, x1) = x0 + x1,
      
      [len](x0) = 2x0 + 4,
      
      [a__len](x0) = 2x0 + 4,
      
      [add](x0, x1) = x0 + x1 + 2,
      
      [a__add](x0, x1) = x0 + x1 + 2,
      
      [from](x0) = 2x0 + 6,
      
      [a__from](x0) = 2x0 + 6,
      
      [fst](x0, x1) = x0 + x1 + 1,
      
      [mark](x0) = x0,
      
      [cons](x0, x1) = x0 + 6,
      
      [s](x0) = 3,
      
      [nil] = 2,
      
      [a__fst](x0, x1) = x0 + x1 + 1,
      
      [0] = 4
     orientation:
      a__add#(0(),X) = X + 1 >= X = mark#(X)
      
      mark#(fst(X1,X2)) = X1 + X2 + 1 >= X2 = mark#(X2)
      
      mark#(fst(X1,X2)) = X1 + X2 + 1 >= X1 = mark#(X1)
      
      mark#(fst(X1,X2)) = X1 + X2 + 1 >= X1 + X2 = a__fst#(mark(X1),mark(X2))
      
      a__fst#(s(X),cons(Y,Z)) = Y + 9 >= Y = mark#(Y)
      
      mark#(from(X)) = 2X + 6 >= X = mark#(X)
      
      mark#(from(X)) = 2X + 6 >= 2X + 4 = a__from#(mark(X))
      
      a__from#(X) = 2X + 4 >= X = mark#(X)
      
      mark#(add(X1,X2)) = X1 + X2 + 2 >= X2 = mark#(X2)
      
      mark#(add(X1,X2)) = X1 + X2 + 2 >= X1 = mark#(X1)
      
      mark#(add(X1,X2)) = X1 + X2 + 2 >= X2 + 1 = a__add#(mark(X1),mark(X2))
      
      mark#(len(X)) = 2X + 4 >= X = mark#(X)
      
      mark#(cons(X1,X2)) = X1 + 6 >= X1 = mark#(X1)
      
      a__fst(0(),Z) = Z + 5 >= 2 = nil()
      
      a__fst(s(X),cons(Y,Z)) = Y + 10 >= Y + 6 = cons(mark(Y),fst(X,Z))
      
      a__from(X) = 2X + 6 >= X + 6 = cons(mark(X),from(s(X)))
      
      a__add(0(),X) = X + 6 >= X = mark(X)
      
      a__add(s(X),Y) = Y + 5 >= 3 = s(add(X,Y))
      
      a__len(nil()) = 8 >= 4 = 0()
      
      a__len(cons(X,Z)) = 2X + 16 >= 3 = s(len(Z))
      
      mark(fst(X1,X2)) = X1 + X2 + 1 >= X1 + X2 + 1 = a__fst(mark(X1),mark(X2))
      
      mark(from(X)) = 2X + 6 >= 2X + 6 = a__from(mark(X))
      
      mark(add(X1,X2)) = X1 + X2 + 2 >= X1 + X2 + 2 = a__add(mark(X1),mark(X2))
      
      mark(len(X)) = 2X + 4 >= 2X + 4 = a__len(mark(X))
      
      mark(0()) = 4 >= 4 = 0()
      
      mark(s(X)) = 3 >= 3 = s(X)
      
      mark(nil()) = 2 >= 2 = nil()
      
      mark(cons(X1,X2)) = X1 + 6 >= X1 + 6 = cons(mark(X1),X2)
      
      a__fst(X1,X2) = X1 + X2 + 1 >= X1 + X2 + 1 = fst(X1,X2)
      
      a__from(X) = 2X + 6 >= 2X + 6 = from(X)
      
      a__add(X1,X2) = X1 + X2 + 2 >= X1 + X2 + 2 = add(X1,X2)
      
      a__len(X) = 2X + 4 >= 2X + 4 = len(X)
     problem:
      DPs:
       
      TRS:
       a__fst(0(),Z) -> nil()
       a__fst(s(X),cons(Y,Z)) -> cons(mark(Y),fst(X,Z))
       a__from(X) -> cons(mark(X),from(s(X)))
       a__add(0(),X) -> mark(X)
       a__add(s(X),Y) -> s(add(X,Y))
       a__len(nil()) -> 0()
       a__len(cons(X,Z)) -> s(len(Z))
       mark(fst(X1,X2)) -> a__fst(mark(X1),mark(X2))
       mark(from(X)) -> a__from(mark(X))
       mark(add(X1,X2)) -> a__add(mark(X1),mark(X2))
       mark(len(X)) -> a__len(mark(X))
       mark(0()) -> 0()
       mark(s(X)) -> s(X)
       mark(nil()) -> nil()
       mark(cons(X1,X2)) -> cons(mark(X1),X2)
       a__fst(X1,X2) -> fst(X1,X2)
       a__from(X) -> from(X)
       a__add(X1,X2) -> add(X1,X2)
       a__len(X) -> len(X)
     Qed