Fixpoint remove_aux {A} (A_eqb : A -> A -> bool) (lst : list A) (x : A) : list A :=
  match lst with
  | [] => []
  | h :: t => 
      if A_eqb h x then t else h :: remove_aux A_eqb t x
  end.

Lemma remove_aux_iff {A} `{EqDec A} : forall lst a a', NoDup lst -> In a' (remove_aux eqb lst a) <-> In a' lst /\ a' <> a.