Definition Irreflexive (U : Set) (R : Relation U) : Prop :=
  forall x : U, ~ R x x.Definition Separating (U : Set) (R : Relation U) : Prop :=
  forall x y z : U, R x y -> R x z \/ R y z.Inductive Apartness (U : Set) (R : Relation U) : Prop :=
    Definition_of_apartness :
      Irreflexive U R -> Separating U R -> Apartness U R.Inductive Apartness (U : Set) (R : Relation U) : Prop :=
    Definition_of_apartness :
      Irreflexive U R -> Separating U R -> Apartness U R.(****************************************************************)

Parameter Point : Set.Parameter Line : Set.Parameter DiPt : Point -> Point -> Prop.Parameter DiLn : Line -> Line -> Prop.Parameter ConLn : Line -> Line -> Prop.Axiom apart_dipt : Apartness Point DiPt.Axiom apart_diln : Apartness Line DiLn.Axiom apart_con : Apartness Line ConLn.

Theorem sym_DiLn : forall x y : Line, DiLn x y -> DiLn y x.