Def (Ln) == if n=0 nil else (Ln-1) @ L fi (recursive)

Thm* Alph:Type, L:Alph*, n:. (Ln) Alph*

Def as @ bs == Case of as; nil bs ; a.as' a.(as' @ bs) (recursive)

Thm* T:Type, as,bs:T*. (as @ bs) T*

Def i=j == if i=j true ; false fi

Thm* i,j:. i=j