fact: Definition equiv_O_sum {A B} : O (A + B) <~> O (O A + O B) := equiv_O_functor_sum (to O A) (to O B).
imports: HoTT.Basics HoTT.Types Equiv.BiInv Extensions HProp HFiber NullHomotopy Limits.Pullback PathAny Colimits.Pushout