Normal-form bisimilarity is a simple. easy-to-use behavioral equivalence that relates terms in $\lambda$-calculi by decomposing their normal forms into bisimilar subterms. Moreover. it typically allows for powerful up-to techniques. https://www.jmannino.com/save-now-3D-Key-Ring-Rainbow-Lorikeet-Pete-Cromer-Wilmot-Harvey-super-grab/