mathlib

db091637 - feat(combinatorics/simple_graph/subgraph): single-vertex and single-edge subgraphs (#16435)

feat(combinatorics/simple_graph/subgraph): single-vertex and single-edge subgraphs (#16435) Constructors for single-vertex and single-edge subgraphs along with some associated properties. Also includes dot notation improvements for `simple_graph.subgraph.adj`. Co-authored-by: Joanna Choules <joannachoules+lean@googlemail.com>