The Runtime Complexity (innermost) of the given CpxTRS could be proven to be BOUNDS(INF, INF).

(0) Obligation:

Runtime Complexity TRS:
The TRS R consists of the following rules:

letexp(x, y) → letexp(Cons(Cons(Nil, Nil), x), y)
goal(x, y) → letexp(x, y)

Rewrite Strategy: INNERMOST

(1) InfiniteLowerBoundProof (EQUIVALENT transformation)

The loop following loop proves infinite runtime complexity:
The rewrite sequence
letexp(x, y) →⁺ letexp(Cons(Cons(Nil, Nil), x), y)
gives rise to a decreasing loop by considering the right hand sides subterm at position [].
The pumping substitution is [ ].
The result substitution is [x / Cons(Cons(Nil, Nil), x)].

(2) BOUNDS(INF, INF)