mathlib

e56b8fea - feat(model_theory/graph): First-order language and theory of graphs (#13720)

feat(model_theory/graph): First-order language and theory of graphs (#13720) Defines `first_order.language.graph`, the language of graphs Defines `first_order.Theory.simple_graph`, the theory of simple graphs Produces models of the theory of simple graphs from simple graphs and vice versa.