Require Import ProofCheckingEuclid.by_def_CongA. Require Import ProofCheckingEuclid.by_def_OnRay_from_neq_A_B. Require Import ProofCheckingEuclid.by_prop_CongA ...

With first-order logic, we can say what it means for a relation to be reflexive, symmetric, transitive, antisymmetric, and so on: We can then use the notions freely. Notice that Lean will unfold the ...