(** Some properties of [is_mill] and [dests_disjoint]. *)
Definition dests_disjoint (m1 m2: moves) : Prop :=
list_disjoint (dests m1) (dests m2).

Lemma dests_disjoint_sym:
forall m1 m2,
dests_disjoint m1 m2 <-> dests_disjoint m2 m1.