Class Prng

crates/kernel/src/subst.rs:1345–1345

Small deterministic xorshift64 PRNG used for property-style tests. Avoids pulling `rand` into the kernel test module.

    let fv_outer_id = FVarId(7);
    let fv_inner_id = FVarId(8);
    let fv_outer = AE::fvar(fv_outer_id, ());
    let fv_inner = AE::fvar(fv_inner_id, ());

    // Open: peel the outer binder around body... actually body itself is a