Lean Kernel Arena / perf/folded-constant-first

The declaration to check compares

tagged (count #n)      and      (false, count #n)

where tagged m = (isZero m, m), so unfolding the left side leaves the same application count #n in both components, forced by isZero in one and plain in the other.

The plain occurrences are identical, for Θ(1); the forced one evaluates count #n once. The order matters for a checker that leaves a constant unfolded once it reduces it: reducing the forced component first replaces count #n by its value on one side, and the plain comparison then faces a folded application against an evaluated one. Here the forcing component comes first; folded-constant-last.lean swaps them, and the ratio between the two files is what that costs.

N=1000 in the Lean source. From Courant and Leroy, POPL 2026, §10, where the two orders cost Rocq 3 × 10⁻⁵ s and 0.078 s.