YES

Problem:
 a__zeros() -> cons(0(),zeros())
 a__U11(tt()) -> tt()
 a__U21(tt()) -> tt()
 a__U31(tt()) -> tt()
 a__U41(tt(),V2) -> a__U42(a__isNatIList(V2))
 a__U42(tt()) -> tt()
 a__U51(tt(),V2) -> a__U52(a__isNatList(V2))
 a__U52(tt()) -> tt()
 a__U61(tt(),L,N) -> a__U62(a__isNat(N),L)
 a__U62(tt(),L) -> s(a__length(mark(L)))
 a__isNat(0()) -> tt()
 a__isNat(length(V1)) -> a__U11(a__isNatList(V1))
 a__isNat(s(V1)) -> a__U21(a__isNat(V1))
 a__isNatIList(V) -> a__U31(a__isNatList(V))
 a__isNatIList(zeros()) -> tt()
 a__isNatIList(cons(V1,V2)) -> a__U41(a__isNat(V1),V2)
 a__isNatList(nil()) -> tt()
 a__isNatList(cons(V1,V2)) -> a__U51(a__isNat(V1),V2)
 a__length(nil()) -> 0()
 a__length(cons(N,L)) -> a__U61(a__isNatList(L),L,N)
 mark(zeros()) -> a__zeros()
 mark(U11(X)) -> a__U11(mark(X))
 mark(U21(X)) -> a__U21(mark(X))
 mark(U31(X)) -> a__U31(mark(X))
 mark(U41(X1,X2)) -> a__U41(mark(X1),X2)
 mark(U42(X)) -> a__U42(mark(X))
 mark(isNatIList(X)) -> a__isNatIList(X)
 mark(U51(X1,X2)) -> a__U51(mark(X1),X2)
 mark(U52(X)) -> a__U52(mark(X))
 mark(isNatList(X)) -> a__isNatList(X)
 mark(U61(X1,X2,X3)) -> a__U61(mark(X1),X2,X3)
 mark(U62(X1,X2)) -> a__U62(mark(X1),X2)
 mark(isNat(X)) -> a__isNat(X)
 mark(length(X)) -> a__length(mark(X))
 mark(cons(X1,X2)) -> cons(mark(X1),X2)
 mark(0()) -> 0()
 mark(tt()) -> tt()
 mark(s(X)) -> s(mark(X))
 mark(nil()) -> nil()
 a__zeros() -> zeros()
 a__U11(X) -> U11(X)
 a__U21(X) -> U21(X)
 a__U31(X) -> U31(X)
 a__U41(X1,X2) -> U41(X1,X2)
 a__U42(X) -> U42(X)
 a__isNatIList(X) -> isNatIList(X)
 a__U51(X1,X2) -> U51(X1,X2)
 a__U52(X) -> U52(X)
 a__isNatList(X) -> isNatList(X)
 a__U61(X1,X2,X3) -> U61(X1,X2,X3)
 a__U62(X1,X2) -> U62(X1,X2)
 a__isNat(X) -> isNat(X)
 a__length(X) -> length(X)

Proof:
 DP Processor:
  DPs:
   a__U41#(tt(),V2) -> a__isNatIList#(V2)
   a__U41#(tt(),V2) -> a__U42#(a__isNatIList(V2))
   a__U51#(tt(),V2) -> a__isNatList#(V2)
   a__U51#(tt(),V2) -> a__U52#(a__isNatList(V2))
   a__U61#(tt(),L,N) -> a__isNat#(N)
   a__U61#(tt(),L,N) -> a__U62#(a__isNat(N),L)
   a__U62#(tt(),L) -> mark#(L)
   a__U62#(tt(),L) -> a__length#(mark(L))
   a__isNat#(length(V1)) -> a__isNatList#(V1)
   a__isNat#(length(V1)) -> a__U11#(a__isNatList(V1))
   a__isNat#(s(V1)) -> a__isNat#(V1)
   a__isNat#(s(V1)) -> a__U21#(a__isNat(V1))
   a__isNatIList#(V) -> a__isNatList#(V)
   a__isNatIList#(V) -> a__U31#(a__isNatList(V))
   a__isNatIList#(cons(V1,V2)) -> a__isNat#(V1)
   a__isNatIList#(cons(V1,V2)) -> a__U41#(a__isNat(V1),V2)
   a__isNatList#(cons(V1,V2)) -> a__isNat#(V1)
   a__isNatList#(cons(V1,V2)) -> a__U51#(a__isNat(V1),V2)
   a__length#(cons(N,L)) -> a__isNatList#(L)
   a__length#(cons(N,L)) -> a__U61#(a__isNatList(L),L,N)
   mark#(zeros()) -> a__zeros#()
   mark#(U11(X)) -> mark#(X)
   mark#(U11(X)) -> a__U11#(mark(X))
   mark#(U21(X)) -> mark#(X)
   mark#(U21(X)) -> a__U21#(mark(X))
   mark#(U31(X)) -> mark#(X)
   mark#(U31(X)) -> a__U31#(mark(X))
   mark#(U41(X1,X2)) -> mark#(X1)
   mark#(U41(X1,X2)) -> a__U41#(mark(X1),X2)
   mark#(U42(X)) -> mark#(X)
   mark#(U42(X)) -> a__U42#(mark(X))
   mark#(isNatIList(X)) -> a__isNatIList#(X)
   mark#(U51(X1,X2)) -> mark#(X1)
   mark#(U51(X1,X2)) -> a__U51#(mark(X1),X2)
   mark#(U52(X)) -> mark#(X)
   mark#(U52(X)) -> a__U52#(mark(X))
   mark#(isNatList(X)) -> a__isNatList#(X)
   mark#(U61(X1,X2,X3)) -> mark#(X1)
   mark#(U61(X1,X2,X3)) -> a__U61#(mark(X1),X2,X3)
   mark#(U62(X1,X2)) -> mark#(X1)
   mark#(U62(X1,X2)) -> a__U62#(mark(X1),X2)
   mark#(isNat(X)) -> a__isNat#(X)
   mark#(length(X)) -> mark#(X)
   mark#(length(X)) -> a__length#(mark(X))
   mark#(cons(X1,X2)) -> mark#(X1)
   mark#(s(X)) -> mark#(X)
  TRS:
   a__zeros() -> cons(0(),zeros())
   a__U11(tt()) -> tt()
   a__U21(tt()) -> tt()
   a__U31(tt()) -> tt()
   a__U41(tt(),V2) -> a__U42(a__isNatIList(V2))
   a__U42(tt()) -> tt()
   a__U51(tt(),V2) -> a__U52(a__isNatList(V2))
   a__U52(tt()) -> tt()
   a__U61(tt(),L,N) -> a__U62(a__isNat(N),L)
   a__U62(tt(),L) -> s(a__length(mark(L)))
   a__isNat(0()) -> tt()
   a__isNat(length(V1)) -> a__U11(a__isNatList(V1))
   a__isNat(s(V1)) -> a__U21(a__isNat(V1))
   a__isNatIList(V) -> a__U31(a__isNatList(V))
   a__isNatIList(zeros()) -> tt()
   a__isNatIList(cons(V1,V2)) -> a__U41(a__isNat(V1),V2)
   a__isNatList(nil()) -> tt()
   a__isNatList(cons(V1,V2)) -> a__U51(a__isNat(V1),V2)
   a__length(nil()) -> 0()
   a__length(cons(N,L)) -> a__U61(a__isNatList(L),L,N)
   mark(zeros()) -> a__zeros()
   mark(U11(X)) -> a__U11(mark(X))
   mark(U21(X)) -> a__U21(mark(X))
   mark(U31(X)) -> a__U31(mark(X))
   mark(U41(X1,X2)) -> a__U41(mark(X1),X2)
   mark(U42(X)) -> a__U42(mark(X))
   mark(isNatIList(X)) -> a__isNatIList(X)
   mark(U51(X1,X2)) -> a__U51(mark(X1),X2)
   mark(U52(X)) -> a__U52(mark(X))
   mark(isNatList(X)) -> a__isNatList(X)
   mark(U61(X1,X2,X3)) -> a__U61(mark(X1),X2,X3)
   mark(U62(X1,X2)) -> a__U62(mark(X1),X2)
   mark(isNat(X)) -> a__isNat(X)
   mark(length(X)) -> a__length(mark(X))
   mark(cons(X1,X2)) -> cons(mark(X1),X2)
   mark(0()) -> 0()
   mark(tt()) -> tt()
   mark(s(X)) -> s(mark(X))
   mark(nil()) -> nil()
   a__zeros() -> zeros()
   a__U11(X) -> U11(X)
   a__U21(X) -> U21(X)
   a__U31(X) -> U31(X)
   a__U41(X1,X2) -> U41(X1,X2)
   a__U42(X) -> U42(X)
   a__isNatIList(X) -> isNatIList(X)
   a__U51(X1,X2) -> U51(X1,X2)
   a__U52(X) -> U52(X)
   a__isNatList(X) -> isNatList(X)
   a__U61(X1,X2,X3) -> U61(X1,X2,X3)
   a__U62(X1,X2) -> U62(X1,X2)
   a__isNat(X) -> isNat(X)
   a__length(X) -> length(X)
  Matrix Interpretation Processor: dim=1
   
   interpretation:
    [a__length#](x0) = x0,
    
    [mark#](x0) = 4x0 + 1,
    
    [a__U62#](x0, x1) = 4x1 + 4,
    
    [a__isNat#](x0) = 4x0 + 4,
    
    [a__U61#](x0, x1, x2) = 3x0 + 4x1 + 4x2 + 2,
    
    [a__U52#](x0) = 4x0,
    
    [a__isNatList#](x0) = 4x0,
    
    [a__U51#](x0, x1) = 4x1 + 2,
    
    [a__U42#](x0) = 0,
    
    [a__isNatIList#](x0) = 4x0 + 6,
    
    [a__U41#](x0, x1) = 6x1 + 7,
    
    [a__U31#](x0) = x0 + 4,
    
    [a__U21#](x0) = x0 + 5,
    
    [a__U11#](x0) = x0,
    
    [a__zeros#] = 0,
    
    [isNat](x0) = x0 + 1,
    
    [U62](x0, x1) = x0 + 4x1 + 6,
    
    [U61](x0, x1, x2) = 3x0 + x1 + 2x2 + 4,
    
    [isNatList](x0) = x0,
    
    [U52](x0) = 4x0 + 3,
    
    [U51](x0, x1) = 2x0 + 4x1 + 1,
    
    [isNatIList](x0) = 2x0 + 4,
    
    [U42](x0) = 2x0 + 1,
    
    [U41](x0, x1) = 4x0 + 4x1 + 4,
    
    [U31](x0) = x0 + 4,
    
    [U21](x0) = x0 + 2,
    
    [U11](x0) = x0 + 1,
    
    [nil] = 1,
    
    [length](x0) = x0 + 2,
    
    [s](x0) = x0 + 1,
    
    [a__length](x0) = x0 + 2,
    
    [mark](x0) = 4x0 + 3,
    
    [a__U62](x0, x1) = x0 + 4x1 + 6,
    
    [a__isNat](x0) = 2x0 + 1,
    
    [a__U61](x0, x1, x2) = 3x0 + 4x1 + 2x2 + 4,
    
    [a__U52](x0) = 4x0 + 3,
    
    [a__isNatList](x0) = x0,
    
    [a__U51](x0, x1) = 2x0 + 7x1 + 1,
    
    [a__U42](x0) = 2x0 + 1,
    
    [a__isNatIList](x0) = 3x0 + 4,
    
    [a__U41](x0, x1) = 4x0 + 6x1 + 5,
    
    [a__U31](x0) = x0 + 4,
    
    [a__U21](x0) = x0 + 2,
    
    [a__U11](x0) = x0 + 4,
    
    [tt] = 1,
    
    [cons](x0, x1) = 4x0 + 7x1 + 3,
    
    [zeros] = 0,
    
    [0] = 0,
    
    [a__zeros] = 3
   orientation:
    a__U41#(tt(),V2) = 6V2 + 7 >= 4V2 + 6 = a__isNatIList#(V2)
    
    a__U41#(tt(),V2) = 6V2 + 7 >= 0 = a__U42#(a__isNatIList(V2))
    
    a__U51#(tt(),V2) = 4V2 + 2 >= 4V2 = a__isNatList#(V2)
    
    a__U51#(tt(),V2) = 4V2 + 2 >= 4V2 = a__U52#(a__isNatList(V2))
    
    a__U61#(tt(),L,N) = 4L + 4N + 5 >= 4N + 4 = a__isNat#(N)
    
    a__U61#(tt(),L,N) = 4L + 4N + 5 >= 4L + 4 = a__U62#(a__isNat(N),L)
    
    a__U62#(tt(),L) = 4L + 4 >= 4L + 1 = mark#(L)
    
    a__U62#(tt(),L) = 4L + 4 >= 4L + 3 = a__length#(mark(L))
    
    a__isNat#(length(V1)) = 4V1 + 12 >= 4V1 = a__isNatList#(V1)
    
    a__isNat#(length(V1)) = 4V1 + 12 >= V1 = a__U11#(a__isNatList(V1))
    
    a__isNat#(s(V1)) = 4V1 + 8 >= 4V1 + 4 = a__isNat#(V1)
    
    a__isNat#(s(V1)) = 4V1 + 8 >= 2V1 + 6 = a__U21#(a__isNat(V1))
    
    a__isNatIList#(V) = 4V + 6 >= 4V = a__isNatList#(V)
    
    a__isNatIList#(V) = 4V + 6 >= V + 4 = a__U31#(a__isNatList(V))
    
    a__isNatIList#(cons(V1,V2)) = 16V1 + 28V2 + 18 >= 4V1 + 4 = a__isNat#(V1)
    
    a__isNatIList#(cons(V1,V2)) = 16V1 + 28V2 + 18 >= 6V2 + 7 = a__U41#(a__isNat(V1),V2)
    
    a__isNatList#(cons(V1,V2)) = 16V1 + 28V2 + 12 >= 4V1 + 4 = a__isNat#(V1)
    
    a__isNatList#(cons(V1,V2)) = 16V1 + 28V2 + 12 >= 4V2 + 2 = a__U51#(a__isNat(V1),V2)
    
    a__length#(cons(N,L)) = 7L + 4N + 3 >= 4L = a__isNatList#(L)
    
    a__length#(cons(N,L)) = 7L + 4N + 3 >= 7L + 4N + 2 = a__U61#(a__isNatList(L),L,N)
    
    mark#(zeros()) = 1 >= 0 = a__zeros#()
    
    mark#(U11(X)) = 4X + 5 >= 4X + 1 = mark#(X)
    
    mark#(U11(X)) = 4X + 5 >= 4X + 3 = a__U11#(mark(X))
    
    mark#(U21(X)) = 4X + 9 >= 4X + 1 = mark#(X)
    
    mark#(U21(X)) = 4X + 9 >= 4X + 8 = a__U21#(mark(X))
    
    mark#(U31(X)) = 4X + 17 >= 4X + 1 = mark#(X)
    
    mark#(U31(X)) = 4X + 17 >= 4X + 7 = a__U31#(mark(X))
    
    mark#(U41(X1,X2)) = 16X1 + 16X2 + 17 >= 4X1 + 1 = mark#(X1)
    
    mark#(U41(X1,X2)) = 16X1 + 16X2 + 17 >= 6X2 + 7 = a__U41#(mark(X1),X2)
    
    mark#(U42(X)) = 8X + 5 >= 4X + 1 = mark#(X)
    
    mark#(U42(X)) = 8X + 5 >= 0 = a__U42#(mark(X))
    
    mark#(isNatIList(X)) = 8X + 17 >= 4X + 6 = a__isNatIList#(X)
    
    mark#(U51(X1,X2)) = 8X1 + 16X2 + 5 >= 4X1 + 1 = mark#(X1)
    
    mark#(U51(X1,X2)) = 8X1 + 16X2 + 5 >= 4X2 + 2 = a__U51#(mark(X1),X2)
    
    mark#(U52(X)) = 16X + 13 >= 4X + 1 = mark#(X)
    
    mark#(U52(X)) = 16X + 13 >= 16X + 12 = a__U52#(mark(X))
    
    mark#(isNatList(X)) = 4X + 1 >= 4X = a__isNatList#(X)
    
    mark#(U61(X1,X2,X3)) = 12X1 + 4X2 + 8X3 + 17 >= 4X1 + 1 = mark#(X1)
    
    mark#(U61(X1,X2,X3)) = 12X1 + 4X2 + 8X3 + 17 >= 12X1 + 4X2 + 4X3 + 11 = a__U61#(mark(X1),X2,X3)
    
    mark#(U62(X1,X2)) = 4X1 + 16X2 + 25 >= 4X1 + 1 = mark#(X1)
    
    mark#(U62(X1,X2)) = 4X1 + 16X2 + 25 >= 4X2 + 4 = a__U62#(mark(X1),X2)
    
    mark#(isNat(X)) = 4X + 5 >= 4X + 4 = a__isNat#(X)
    
    mark#(length(X)) = 4X + 9 >= 4X + 1 = mark#(X)
    
    mark#(length(X)) = 4X + 9 >= 4X + 3 = a__length#(mark(X))
    
    mark#(cons(X1,X2)) = 16X1 + 28X2 + 13 >= 4X1 + 1 = mark#(X1)
    
    mark#(s(X)) = 4X + 5 >= 4X + 1 = mark#(X)
    
    a__zeros() = 3 >= 3 = cons(0(),zeros())
    
    a__U11(tt()) = 5 >= 1 = tt()
    
    a__U21(tt()) = 3 >= 1 = tt()
    
    a__U31(tt()) = 5 >= 1 = tt()
    
    a__U41(tt(),V2) = 6V2 + 9 >= 6V2 + 9 = a__U42(a__isNatIList(V2))
    
    a__U42(tt()) = 3 >= 1 = tt()
    
    a__U51(tt(),V2) = 7V2 + 3 >= 4V2 + 3 = a__U52(a__isNatList(V2))
    
    a__U52(tt()) = 7 >= 1 = tt()
    
    a__U61(tt(),L,N) = 4L + 2N + 7 >= 4L + 2N + 7 = a__U62(a__isNat(N),L)
    
    a__U62(tt(),L) = 4L + 7 >= 4L + 6 = s(a__length(mark(L)))
    
    a__isNat(0()) = 1 >= 1 = tt()
    
    a__isNat(length(V1)) = 2V1 + 5 >= V1 + 4 = a__U11(a__isNatList(V1))
    
    a__isNat(s(V1)) = 2V1 + 3 >= 2V1 + 3 = a__U21(a__isNat(V1))
    
    a__isNatIList(V) = 3V + 4 >= V + 4 = a__U31(a__isNatList(V))
    
    a__isNatIList(zeros()) = 4 >= 1 = tt()
    
    a__isNatIList(cons(V1,V2)) = 12V1 + 21V2 + 13 >= 8V1 + 6V2 + 9 = a__U41(a__isNat(V1),V2)
    
    a__isNatList(nil()) = 1 >= 1 = tt()
    
    a__isNatList(cons(V1,V2)) = 4V1 + 7V2 + 3 >= 4V1 + 7V2 + 3 = a__U51(a__isNat(V1),V2)
    
    a__length(nil()) = 3 >= 0 = 0()
    
    a__length(cons(N,L)) = 7L + 4N + 5 >= 7L + 2N + 4 = a__U61(a__isNatList(L),L,N)
    
    mark(zeros()) = 3 >= 3 = a__zeros()
    
    mark(U11(X)) = 4X + 7 >= 4X + 7 = a__U11(mark(X))
    
    mark(U21(X)) = 4X + 11 >= 4X + 5 = a__U21(mark(X))
    
    mark(U31(X)) = 4X + 19 >= 4X + 7 = a__U31(mark(X))
    
    mark(U41(X1,X2)) = 16X1 + 16X2 + 19 >= 16X1 + 6X2 + 17 = a__U41(mark(X1),X2)
    
    mark(U42(X)) = 8X + 7 >= 8X + 7 = a__U42(mark(X))
    
    mark(isNatIList(X)) = 8X + 19 >= 3X + 4 = a__isNatIList(X)
    
    mark(U51(X1,X2)) = 8X1 + 16X2 + 7 >= 8X1 + 7X2 + 7 = a__U51(mark(X1),X2)
    
    mark(U52(X)) = 16X + 15 >= 16X + 15 = a__U52(mark(X))
    
    mark(isNatList(X)) = 4X + 3 >= X = a__isNatList(X)
    
    mark(U61(X1,X2,X3)) = 12X1 + 4X2 + 8X3 + 19 >= 12X1 + 4X2 + 2X3 + 13 = a__U61(mark(X1),X2,X3)
    
    mark(U62(X1,X2)) = 4X1 + 16X2 + 27 >= 4X1 + 4X2 + 9 = a__U62(mark(X1),X2)
    
    mark(isNat(X)) = 4X + 7 >= 2X + 1 = a__isNat(X)
    
    mark(length(X)) = 4X + 11 >= 4X + 5 = a__length(mark(X))
    
    mark(cons(X1,X2)) = 16X1 + 28X2 + 15 >= 16X1 + 7X2 + 15 = cons(mark(X1),X2)
    
    mark(0()) = 3 >= 0 = 0()
    
    mark(tt()) = 7 >= 1 = tt()
    
    mark(s(X)) = 4X + 7 >= 4X + 4 = s(mark(X))
    
    mark(nil()) = 7 >= 1 = nil()
    
    a__zeros() = 3 >= 0 = zeros()
    
    a__U11(X) = X + 4 >= X + 1 = U11(X)
    
    a__U21(X) = X + 2 >= X + 2 = U21(X)
    
    a__U31(X) = X + 4 >= X + 4 = U31(X)
    
    a__U41(X1,X2) = 4X1 + 6X2 + 5 >= 4X1 + 4X2 + 4 = U41(X1,X2)
    
    a__U42(X) = 2X + 1 >= 2X + 1 = U42(X)
    
    a__isNatIList(X) = 3X + 4 >= 2X + 4 = isNatIList(X)
    
    a__U51(X1,X2) = 2X1 + 7X2 + 1 >= 2X1 + 4X2 + 1 = U51(X1,X2)
    
    a__U52(X) = 4X + 3 >= 4X + 3 = U52(X)
    
    a__isNatList(X) = X >= X = isNatList(X)
    
    a__U61(X1,X2,X3) = 3X1 + 4X2 + 2X3 + 4 >= 3X1 + X2 + 2X3 + 4 = U61(X1,X2,X3)
    
    a__U62(X1,X2) = X1 + 4X2 + 6 >= X1 + 4X2 + 6 = U62(X1,X2)
    
    a__isNat(X) = 2X + 1 >= X + 1 = isNat(X)
    
    a__length(X) = X + 2 >= X + 2 = length(X)
   problem:
    DPs:
     
    TRS:
     a__zeros() -> cons(0(),zeros())
     a__U11(tt()) -> tt()
     a__U21(tt()) -> tt()
     a__U31(tt()) -> tt()
     a__U41(tt(),V2) -> a__U42(a__isNatIList(V2))
     a__U42(tt()) -> tt()
     a__U51(tt(),V2) -> a__U52(a__isNatList(V2))
     a__U52(tt()) -> tt()
     a__U61(tt(),L,N) -> a__U62(a__isNat(N),L)
     a__U62(tt(),L) -> s(a__length(mark(L)))
     a__isNat(0()) -> tt()
     a__isNat(length(V1)) -> a__U11(a__isNatList(V1))
     a__isNat(s(V1)) -> a__U21(a__isNat(V1))
     a__isNatIList(V) -> a__U31(a__isNatList(V))
     a__isNatIList(zeros()) -> tt()
     a__isNatIList(cons(V1,V2)) -> a__U41(a__isNat(V1),V2)
     a__isNatList(nil()) -> tt()
     a__isNatList(cons(V1,V2)) -> a__U51(a__isNat(V1),V2)
     a__length(nil()) -> 0()
     a__length(cons(N,L)) -> a__U61(a__isNatList(L),L,N)
     mark(zeros()) -> a__zeros()
     mark(U11(X)) -> a__U11(mark(X))
     mark(U21(X)) -> a__U21(mark(X))
     mark(U31(X)) -> a__U31(mark(X))
     mark(U41(X1,X2)) -> a__U41(mark(X1),X2)
     mark(U42(X)) -> a__U42(mark(X))
     mark(isNatIList(X)) -> a__isNatIList(X)
     mark(U51(X1,X2)) -> a__U51(mark(X1),X2)
     mark(U52(X)) -> a__U52(mark(X))
     mark(isNatList(X)) -> a__isNatList(X)
     mark(U61(X1,X2,X3)) -> a__U61(mark(X1),X2,X3)
     mark(U62(X1,X2)) -> a__U62(mark(X1),X2)
     mark(isNat(X)) -> a__isNat(X)
     mark(length(X)) -> a__length(mark(X))
     mark(cons(X1,X2)) -> cons(mark(X1),X2)
     mark(0()) -> 0()
     mark(tt()) -> tt()
     mark(s(X)) -> s(mark(X))
     mark(nil()) -> nil()
     a__zeros() -> zeros()
     a__U11(X) -> U11(X)
     a__U21(X) -> U21(X)
     a__U31(X) -> U31(X)
     a__U41(X1,X2) -> U41(X1,X2)
     a__U42(X) -> U42(X)
     a__isNatIList(X) -> isNatIList(X)
     a__U51(X1,X2) -> U51(X1,X2)
     a__U52(X) -> U52(X)
     a__isNatList(X) -> isNatList(X)
     a__U61(X1,X2,X3) -> U61(X1,X2,X3)
     a__U62(X1,X2) -> U62(X1,X2)
     a__isNat(X) -> isNat(X)
     a__length(X) -> length(X)
   Qed