(VAR L N V V1 V2 X X1 X2 X3 )
(RULES 
        active(zeros) -> mark(cons(0, zeros))
        active(U11(tt)) -> mark(tt)
        active(U21(tt)) -> mark(tt)
        active(U31(tt)) -> mark(tt)
        active(U41(tt, V2)) -> mark(U42(isNatIList(V2)))
        active(U42(tt)) -> mark(tt)
        active(U51(tt, V2)) -> mark(U52(isNatList(V2)))
        active(U52(tt)) -> mark(tt)
        active(U61(tt, L, N)) -> mark(U62(isNat(N), L))
        active(U62(tt, L)) -> mark(s(length(L)))
        active(isNat(0)) -> mark(tt)
        active(isNat(length(V1))) -> mark(U11(isNatList(V1)))
        active(isNat(s(V1))) -> mark(U21(isNat(V1)))
        active(isNatIList(V)) -> mark(U31(isNatList(V)))
        active(isNatIList(zeros)) -> mark(tt)
        active(isNatIList(cons(V1, V2))) -> mark(U41(isNat(V1), V2))
        active(isNatList(nil)) -> mark(tt)
        active(isNatList(cons(V1, V2))) -> mark(U51(isNat(V1), V2))
        active(length(nil)) -> mark(0)
        active(length(cons(N, L))) -> mark(U61(isNatList(L), L, N))
        mark(zeros) -> active(zeros)
        mark(cons(X1, X2)) -> active(cons(mark(X1), X2))
        mark(0) -> active(0)
        mark(U11(X)) -> active(U11(mark(X)))
        mark(tt) -> active(tt)
        mark(U21(X)) -> active(U21(mark(X)))
        mark(U31(X)) -> active(U31(mark(X)))
        mark(U41(X1, X2)) -> active(U41(mark(X1), X2))
        mark(U42(X)) -> active(U42(mark(X)))
        mark(isNatIList(X)) -> active(isNatIList(X))
        mark(U51(X1, X2)) -> active(U51(mark(X1), X2))
        mark(U52(X)) -> active(U52(mark(X)))
        mark(isNatList(X)) -> active(isNatList(X))
        mark(U61(X1, X2, X3)) -> active(U61(mark(X1), X2, X3))
        mark(U62(X1, X2)) -> active(U62(mark(X1), X2))
        mark(isNat(X)) -> active(isNat(X))
        mark(s(X)) -> active(s(mark(X)))
        mark(length(X)) -> active(length(mark(X)))
        mark(nil) -> active(nil)
        cons(mark(X1), X2) -> cons(X1, X2)
        cons(X1, mark(X2)) -> cons(X1, X2)
        cons(active(X1), X2) -> cons(X1, X2)
        cons(X1, active(X2)) -> cons(X1, X2)
        U11(mark(X)) -> U11(X)
        U11(active(X)) -> U11(X)
        U21(mark(X)) -> U21(X)
        U21(active(X)) -> U21(X)
        U31(mark(X)) -> U31(X)
        U31(active(X)) -> U31(X)
        U41(mark(X1), X2) -> U41(X1, X2)
        U41(X1, mark(X2)) -> U41(X1, X2)
        U41(active(X1), X2) -> U41(X1, X2)
        U41(X1, active(X2)) -> U41(X1, X2)
        U42(mark(X)) -> U42(X)
        U42(active(X)) -> U42(X)
        isNatIList(mark(X)) -> isNatIList(X)
        isNatIList(active(X)) -> isNatIList(X)
        U51(mark(X1), X2) -> U51(X1, X2)
        U51(X1, mark(X2)) -> U51(X1, X2)
        U51(active(X1), X2) -> U51(X1, X2)
        U51(X1, active(X2)) -> U51(X1, X2)
        U52(mark(X)) -> U52(X)
        U52(active(X)) -> U52(X)
        isNatList(mark(X)) -> isNatList(X)
        isNatList(active(X)) -> isNatList(X)
        U61(mark(X1), X2, X3) -> U61(X1, X2, X3)
        U61(X1, mark(X2), X3) -> U61(X1, X2, X3)
        U61(X1, X2, mark(X3)) -> U61(X1, X2, X3)
        U61(active(X1), X2, X3) -> U61(X1, X2, X3)
        U61(X1, active(X2), X3) -> U61(X1, X2, X3)
        U61(X1, X2, active(X3)) -> U61(X1, X2, X3)
        U62(mark(X1), X2) -> U62(X1, X2)
        U62(X1, mark(X2)) -> U62(X1, X2)
        U62(active(X1), X2) -> U62(X1, X2)
        U62(X1, active(X2)) -> U62(X1, X2)
        isNat(mark(X)) -> isNat(X)
        isNat(active(X)) -> isNat(X)
        s(mark(X)) -> s(X)
        s(active(X)) -> s(X)
        length(mark(X)) -> length(X)
        length(active(X)) -> length(X)
        
)