(* B3 : Given three distinct points on a line, one and only one of them is between the other two. *)

(* Specifications: *)
Lemma B3a : forall A B C,
A <> B ->
B <> C ->
C <> A ->
Collinear A B C ->
Between A B C \/ Between B C A \/ Between C A B.
Lemma B3b : forall A B C,
Between A B C ->
~Between B A C.
Lemma B3c : forall A B C,
Between A B C ->
~Between A C B.