let R be the TRS under consideration

diff(_1,_2) -> if(leq(_1,_2),0,s(diff(p(_1),_2))) is in elim_R(R)
let l be the left-hand side and r be the right-hand side of this rule
let p = 2.0
let theta = {}
let theta' = {_1/p(_1), _2/_2}
we have r|p = diff(p(_1),_2) and
theta'(theta(l)) = theta(r|p)
so, theta(l) = diff(_1,_2) 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