fact: Definition PO_flattening : PO (functor_sigma f f0) (functor_sigma g g0) <~> exists x, POCase_P x.
imports: Basics Types Diagrams.Diagram Diagrams.DDiagram Diagrams.Span Diagrams.Cocone Colimits.Colimit Colimits.Colimit_Pushout Colimits.Colimit_Flattening