let R be the TRS under consideration

a__U11(tt,_1) -> a__U12(tt,_1) is in elim_R(R)
let l0 be the left-hand side of this rule
p0 = epsilon is a position in l0
we have l0|p0 = a__U11(tt,_1)
a__length(cons(_2,_3)) -> a__U11(tt,_3) is in R
let r'0 be the right-hand side of this rule
theta0 = {_1/_3} is a mgu of l0|p0 and r'0

==> a__length(cons(_1,_2)) -> a__U12(tt,_2) is in EU_R^1
let l1 be the left-hand side of this rule
p1 = 0 is a position in l1
we have l1|p1 = cons(_1,_2)
a__zeros -> cons(0,zeros) is in R
let r'1 be the right-hand side of this rule
theta1 = {_1/0, _2/zeros} is a mgu of l1|p1 and r'1

==> a__length(a__zeros) -> a__U12(tt,zeros) is in EU_R^2
let l2 be the left-hand side of this rule
p2 = 0 is a position in l2
we have l2|p2 = a__zeros
mark(zeros) -> a__zeros is in R
let r'2 be the right-hand side of this rule
theta2 = {} is a mgu of l2|p2 and r'2

==> a__length(mark(zeros)) -> a__U12(tt,zeros) is in EU_R^3
let r3 be the right-hand side of this rule
p3 = epsilon is a position in r3
we have r3|p3 = a__U12(tt,zeros)
a__U12(tt,_1) -> s(a__length(mark(_1))) is in R
let l'3 be the left-hand side of this rule
theta3 = {_1/zeros} is a mgu of r3|p3 and l'3

==> a__length(mark(zeros)) -> s(a__length(mark(zeros))) is in EU_R^4
let l be the left-hand side and r be the right-hand side of this rule
let p = 0
let theta = {}
let theta' = {}
we have r|p = a__length(mark(zeros)) and
theta'(theta(l)) = theta(r|p)
so, theta(l) = a__length(mark(zeros)) is non-terminating w.r.t. R

Termination disproved by the forward+backward process
proof stopped at iteration i=4, depth k=2
124 rule(s) generated