Require Coq.Lists.List.
Require Coq.Sets.Ensembles.
Global Set Implicit Arguments.
Delimit Scope comp_scope with comp.
Inductive Comp : Type -> Type :=
| Return : forall A, A -> Comp A
| Bind : forall A B, Comp A -> (A -> Comp B) -> Comp B
| Pick : forall A, Ensemble A -> Comp A.
Notation ret := Return.
Notation "x <- y ; z" := (Bind y%comp (fun x => z%comp))
(at level 81, right associativity,
format "'[v' x  <-  y ; '/' z ']'") : comp_scope.
Open Scope comp.

Axiom elements : forall {A} (ls : list A), Ensemble A.
Axiom to_list : forall {A} (S : Ensemble A), Comp (list A).
Axiom finite_set_handle_cardinal : refine (ret 0) (ret 0).