Def (+f)(l) == Case of l; nil nil ; h.l' [h; f(h)/ (+f)(l')] (recursive)

Thm* St:Type, l:St*, f:(StSt). (+f)(l) St*

Def P Q == (P Q) & (P Q)

Thm* A,B:Prop. (A B) Prop

Def mem_f(T;a;bs) == Case of bs; nil False ; b.bs' b = a T mem_f(T;a;bs') (recursive)

Thm* T:Type, a:T, bs:T*. mem_f(T;a;bs) Prop

Def P Q == Q P

Thm* A,B:Prop. (A B) Prop