mathlib

5d7efa03 - feat(combinatorics/quiver): define quivers (#6680)

feat(combinatorics/quiver): define quivers (#6680) Define quivers (a very permissive notion of graph), subquivers, paths and arborescences, which are like rooted trees. This PR comes from https://github.com/dwarn/nielsen-schreier-2 .