mathlib3

d521b2bc - feat(algebraic_topology/simplex_category): epi and monos in the simplex category (#8101)

feat(algebraic_topology/simplex_category): epi and monos in the simplex category (#8101) Characterize epimorphisms and monomorphisms in `simplex_category` in terms of the function they represent. Add lemmas about their behavior on length of objects. Co-authored-by: Robin Carlier <57142648+robin-carlier@users.noreply.github.com>