Rob, thank you very much for answering my questions!  That was smart
of you to notice this problem, which I have worried about:

   your axioms seem to be loose about whether `(A, B) same_side l`
   holds if A or B is on the line l.

I like your EquivRelOn R P solution (thanks for the correction), or we
can even define EquivRelOFF R I.  I have two more questions.

1) Can you tell me why this didn't work? 

let SameSideEquivalenceRelation_THM = thm `;
  thus Reflexive_Property /\ Symmetric_Property /\ Transitive_Property
  proof
  qed by SameSideReflexiveRelation_THM, SameSideTransitiveRelation_THM,
  SameSideTransitiveRelation_THM, Reflexive_relation_DEF,
  Symmetric_relation_DEF, Transitive_relation_DEF `;;

I just got the error ::#2 inference time-out.  I imagine I'll have the
same problem with your code.  Maybe this is the wrong way to go about
proving that some relation is an equivalence relation.

2) I wrote a new definition I have a definition

let InteriorAngle_DEF = new_definition
  `!A O B. int_angle(A, O, B) = if Collinear(A, O, B) then {} else 
   {P | ~(P IN line(O, A)) /\ ~(P IN line(O, B)) /\ 
   P,B same_side line(O, A) /\ P,A same_side line(O, B)}`;;

I don't if it's working yet, but I would like to simplify this with a
variable declaration and then substitution 

a = line(O, A)
b = line(O, B)

You must know how to do that. 

-- 
Best,
Bill 

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