Section T17.
Context `{T2D:Tarski_2D}.
Context `{TE:@Tarski_euclidean Tn TnEQD}.
Definition O := PA.
Definition E := PB.
Definition E' := PC.

Lemma pythrel_null : forall O E E' A B, PythRel O E E' A B O -> A = O /\ B = O.