mathlib

b0f585c5 - feat(combinatorics/simple_graph/inc_matrix): Incidence matrix (#10867)

feat(combinatorics/simple_graph/inc_matrix): Incidence matrix (#10867) Define the incidence matrix of a simple graph and prove the basics, including some stuff about matrix multiplication. Co-authored-by: Gabriel Moise <gabriel.moise@stcatz.ox.ac.uk> Co-authored-by: Kyle Miller <kmill31415@gmail.com> Co-authored-by: Kyle Miller <kmill31415@gmail.com>