Instance set_compat2 (A : Type) x dom :
Proper (Logic.eq ==> @eq A dom dom ==> full_relation ==> @eq A dom dom) (@set A dom x).
Instance add_compat2 (A : Type) x dom :
Proper (Logic.eq ==> @eq A dom dom ==> @eq A (Dom.add x dom) (Dom.add x dom)) (@add A dom x).
Instance remove_compat2 (A : Type) x dom :
Proper (@eq A dom dom ==> @eq A (Dom.remove x dom) (Dom.remove x dom)) (@remove A dom x).

Theorem combine_None : forall A B C (f : A -> B -> C) g₁ g₂ dom₁ dom₂ (m₁ : t A dom₁) (m₂ : t B dom₂) x,
find x (combine f g₁ g₂ m₁ m₂) = None <-> find x m₁ = None ∧ find x m₂ = None.