Section Book_1_part_4.
(** * Proposition 31 Through a given point to draw a straight line parallel to a given straight line. *)
(** # <div style="width:748px;height:397px;display:block" id="applet_container31"></div> # **)
Context `{TnEQD:Tarski_neutral_dimensionless_with_decidable_point_equality}.

Lemma prop_31 : forall A B C, B <> C -> exists E, Par B C A E.