let R be the TRS under consideration

fib1(_1,_2) -> cons(_1,fib1(_2,add(_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 = 1
let theta = {}
let theta' = {_1/_2, _2/add(_1,_2)}
we have r|p = fib1(_2,add(_1,_2)) and
theta'(theta(l)) = theta(r|p)
so, theta(l) = fib1(_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