Require Export GeoCoq.Elements.OriginalProofs.lemma_parallelsymmetric.
Require Export GeoCoq.Elements.OriginalProofs.lemma_parallelflip.
Section Euclid.
Context `{Ax1:euclidean_neutral}.

Lemma lemma_PGsymmetric :
forall A B C D,
PG A B C D ->
PG C D A B.