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

Lemma pythrel_not_null : forall O E E' A B C, C <> O -> PythRel O E E' A B C -> A <> O \/ B <> O.