let R be the TRS under consideration

fact(_1) -> if(zero(_1),s(0),prod(_1,fact(p(_1)))) is in elim_R(R)
let l be the left-hand side and r be the right-hand side of this rule
let p = 2.1
let theta = {}
let theta' = {_1/p(_1)}
we have r|p = fact(p(_1)) and
theta'(theta(l)) = theta(r|p)
so, theta(l) = fact(_1) is non-terminating w.r.t. R

Termination disproved by the forward process
proof stopped at iteration i=0, depth k=0
1 rule(s) generated