tp: type. %name tp T.

arrow: tp -> tp -> tp.
forall: (tp -> tp) -> tp.

exp: tp -> type. %name exp E.

lam: (exp T1 -> exp T2) -> exp (arrow T1 T2).
app: exp (arrow T1 T2) -> exp T1 -> exp T2.

tlam: ({t:tp} exp (F t)) -> exp (forall F).
tapp: exp (forall F) -> {t:tp} (exp (F t)).

eval: exp T -> exp T -> type.

ev_lam: eval (lam F) (lam F).
ev_app: eval (app E1 E2) V
        <- eval E1 (lam F1)
        <- eval E2 V2
        <- eval (F1 V2) V.

eval_tlam: eval (tlam F) (tlam F).
eval_tapp: eval (tapp E T) V
           <- eval E (tlam F)
           <- eval (F T) V.