and haves. The proof can be tactical or structured. Generally, we tend to use structured proofs to sketch the main argument and resort to tactical proofs for proving subgoals or uninteresting intermediate the demonstration file associated with this chapter. We con- tent ourselves with an informal proof sketch: The proof is by rule induction over (S, s) ⇒■ ⇒ l. Case Skip: To have (skip, s) ⇒■ ⇒ l or (skip

This can be extremely useful when constructing proofs incrementally. You can start typing a proof sketch, using either sorry or an underscore for details you intend to fill in later. Assuming the proof

shows that the law of the excluded middle follows from classical.choice, propext, and funext. We sketch the proof that is found in the standard library. First, we import the necessary axioms, fix a parameter