Definition isGe (c: comparison) := match c with Lt => False | _ => True end.Definition isGeq cc := match cc with Lt => false | _ => true end.Fixpoint insert {A: Type} (cmp: A -> A -> comparison) (a: A) (l: list A)
  : list A:=
  match l with
  | h :: t => if isGeq (cmp a h)
                 then a :: l
                 else h :: (insert cmp a t)
  | nil => a :: nil
  end.Fixpoint rsort {A: Type} (cmp: A -> A -> comparison) (l: list A) : list A :=
  match l with nil => nil | h::t => insert cmp h (rsort cmp t)
  end.Fixpoint insert_uniq {A: Type} (cmp: A -> A -> comparison) (a: A) (l: list A)
  : list A:=
  match l with
  | h :: t => match cmp a h with
              | Eq => l
              | Gt => a :: l
              | Lt => h :: (insert_uniq cmp a t)
              end
  | nil => a::nil
  end.Fixpoint rsort_uniq {A: Type} (cmp: A -> A -> comparison) (l: list A)
  : list A :=
  match l with nil => nil | h::t => insert_uniq cmp h (rsort_uniq cmp t)
  end.

Lemma perm_insert {T} cmp (a : T) l : Permutation (insert cmp a l) (a :: l).