Yes, this may be the core problem: the default built-in miz3 prover
   doesn't know much about sets [...] SET_TAC [...] it may make sense
   for us to improve the default miz3 prover

Thanks, John!  Maybe we don't have to improve miz3 for my set theory
needs, but instead, I need to rewrite some definitions.  Details:

1) Searching for SET_TAC , and found new set theorems IN_SING,
SING_SUBSET, MEMBER_NOT_EMPTY in sets.ml and got rid of three of mine!

2) sets.ml shows how DELETE & IN_DELETE (which I use in my code) work:

parse_as_infix("DELETE",(21,"left"));;

let DELETE = new_definition
  `s DELETE x = {y:A | y IN s /\ ~(y = x)}`;;

let IN_DELETE = prove
 (`!s. !x:A. !y. x IN (s DELETE y) <=> x IN s /\ ~(x = y)`,
  REWRITE_TAC[IN_ELIM_THM; DELETE]);;

3) Why can't I do that?  I prove theorems ConverseCrossbar_THM &
WholeRayInterior_THM (code included below) using

let Ray_DEF = new_definition
  `!A B X.  ray(X, A, B) <=> ~(A = B) /\ Collinear(A, B, X) /\ ~ Between(X, A, 
B)`;;

let InteriorAngle_DEF = new_definition
 `!A O B P. 
  int_angle(P, A, O, B) <=> ~ Collinear(A, O, B) /\ ?a b. 
  O IN a /\ A IN a /\ O IN b /\ B IN b /\
  ~(P IN a) /\ ~(P IN b) /\ P,B same_side a /\ P,A same_side b`;;

I'm really happy these two proofs worked, because now I know I can
code up my Hilbert axiom paper.  But I'd prefer (change ray to rag):

let Rag_DEF = new_definition
  `!A B.  rag(A, B) = if (A = B) {} else {X | Collinear(A, B, X) /\ ~ 
Between(X, A, B)`;;

let IN_RAG = prove
 (`!A. !X:point. !B. X IN rag(A, B) <=> ~(A = B)  /\ Collinear(A, B, X) /\ ~ 
Between(X, A, B)`,
  REWRITE_TAC[IN_ELIM_THM; DELETE]);;

Ray_DEF worked but IN_RAY gave the error message 

  Exception: Failure "term after binary operator expected".
#       Exception: Failure "TAC_PROOF: Unsolved goals".

I'm so ignorant, I'm not surprised, so I tried a stunt from sets.ml:

let IN_RAG = prove
 (`!A. !X:point. !B. X IN rag(A, B) <=> ~(A = B)  /\ Collinear(A, B, X) /\ ~ 
Between(X, A, B)`,
  REWRITE_TAC[IN_ELIM_THM; DELETE] THEN SET_TAC[]);;

and on a fast 64-bit machine I got the error:
    
0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..0..Exception:
 Failure "solve_goal: Too deep".

I tried other stunts from sets.ml, but nothing worked.  It was fun
trying some actual HOL Light tactics, even in complete ignorance!

-- 
Best,
Bill 

(* Paste in these 2 commands:
   cd ~/hol_light; ocaml
   #use "hol.ml";; 
   then paste in the following file*)

(* ================================================================= *)
(* HOL Light Hilbert geometry axiomatic proofs. *)
(* ================================================================= *)

(* Thanks to Mizar folks who wrote an influential language I was able
   to learn, Freek Wiedijk, who wrote the miz3 port of Mizar to HOL
   Light, and especially John Harrison, who came up with the entire
   framework here of porting my axiomatic proofs to HOL Light.       *)


new_type("point",0);;
new_type_abbrev("line",`:point->bool`);;
new_constant("Between",`:point#point#point->bool`);;
new_constant("===",`:point#point->point#point->bool`);;
parse_as_infix("===",(12, "right"));;
parse_as_infix("cong",(12, "right"));;
parse_as_infix("same_side",(12, "right"));;

let cong_DEF = new_definition
 `A,B,C cong X,Y,Z <=>
   A,B === X,Y /\ A,C === X,Z /\ B,C === Y,Z`;;

let is_ordered_DEF = new_definition
 `is_ordered (A,B,C,D) <=>
  Between (A,B,C) /\ Between (A,B,D) /\ Between (A,C,D) /\ Between (B,C,D)`;;

let Collinear_DEF = new_definition
  `Collinear(A, B, C)  <=> 
  ?l:line. A IN l /\ B IN l /\ C IN l`;;

let Twiddle_DEF = new_definition
  `Twiddle A l B <=> 
  ~(?X. (X IN l) /\ Between(A, X, B))`;;

let same_side_DEF = new_definition
  `A,B same_side l <=> 
  ~(?X. (X IN l) /\ Between(A, X, B))`;;

let Reflexive_relation_DEF = new_definition
  `Reflexive_Property <=> 
  !l:line A:point. ~(A IN l) ==> A,A same_side l`;;

let Symmetric_relation_DEF = new_definition
  `Symmetric_Property <=> 
  !l:line A:point B:point. ~(A IN l) /\ ~(B IN l) ==> 
  A,B same_side l ==> B,A same_side l`;;

let Transitive_relation_DEF = new_definition
  `Transitive_Property <=> 
  !l:line A:point B:point C:point. 
  ~(A IN l) /\ ~(B IN l) /\ ~(C IN l) ==> 
  A,B same_side l /\ B,C same_side l ==> A,C same_side l`;;

let Ray_DEF = new_definition
  `!A B X.  ray(X, A, B) <=> ~(A = B) /\ Collinear(A, B, X) /\ ~ Between(X, A, 
B)`;;

let Angle_DEF = new_definition
  `!A O B. Angle(A, O, B) = if Collinear(A, O, B) then {} else 
  {ray(O, A), ray(O, B)}`;;

let InteriorAngle_DEF = new_definition
 `!A O B P. 
  int_angle(P, A, O, B) <=> ~ Collinear(A, O, B) /\ ?a b. 
  O IN a /\ A IN a /\ O IN b /\ B IN b /\
  ~(P IN a) /\ ~(P IN b) /\ P,B same_side a /\ P,A same_side b`;;


(* ------------------------------------------------------------------------- *)
(* The axioms.                                                               *)
(* ------------------------------------------------------------------------- *)

let I1 = new_axiom
  `!A B.  ~(A = B) ==> ?! l. A IN l /\ B IN l`;;

let I2 = new_axiom
  `!l. ? A B. A IN l /\ B IN l /\ ~(A = B)`;;

let I3 = new_axiom
  `?A:point B:point C:point. ~(A = B) /\ ~(A = C) /\ ~(B = C) /\ ~ Collinear(A, 
B, C)`;;

let B1 = new_axiom
  `! A B C. Between(A, B, C) ==> ~(A = B) /\ ~(A = C) /\ ~(B = C) /\ 
            Between(C, B, A) /\ Collinear(A, B, C)`;;

let B2 = new_axiom
  `! A B. ~(A = B) ==> ?C. Between(A, B, C)`;;

let B3 = new_axiom
  `!A B C. ~(A = B) /\ ~(A = C) /\ ~(B = C) /\ Collinear(A, B, C)
    ==> (Between(A, B, C) \/ Between(B, C, A) \/ Between(C, A, B)) /\
        ~(Between(A, B, C) /\ Between(B, C, A)) /\ 
        ~(Between(A, B, C) /\ Between(C, A, B)) /\ 
        ~(Between(B, C, A) /\ Between(C, A, B))`;;

let B4 = new_axiom
  `!l A B C. ~ Collinear(A, B, C) ==> 
   !l. ~(A IN l) /\ ~(B IN l) /\ ~(C IN l) ==> 
   (?X. X IN l /\ Between(A, X, C)) ==> 
   (?Y. Y IN l /\ Between(A, Y, B)) \/ 
   (?Y. Y IN l /\ Between(B, Y, C))`;;

#load "unix.cma";;
loadt "miz3/miz3.ml";;

horizon := 0;; 


let BiggerThanSingleton_THM = thm `;
  let p be A->bool;
  let x be A;
  assume x IN p                                                         [H1];
  assume ~(p = {x})                                                     [H2];
  thus ?a . a IN p /\ ~(a = x)

  proof
     {x} SUBSET p by H1, SING_SUBSET;
    ~(p SUBSET {x}) by -,  H2, SUBSET_ANTISYM;
    consider a such that 
    a IN p /\ ~(a IN {x})                       [X1] by -, SUBSET;
     ~(a = x) by -, IN_SING;
  qed by -, X1`;;
 

let DisjointOneNotOther_THM = thm `;
  let x be A;
  let l m be A->bool;
  assume l INTER m = {}                                                 [H1];
  assume x IN m                                                         [H2];
  thus ~(x IN l)

  proof
    assume (x IN l);
    x IN l INTER m by -, H2, IN_INTER;
    F by -, NOT_IN_EMPTY, H1;
  qed by -`;;


let IntersectionSingletonOneNotOther_THM = thm `;
  let e x be A;
  let l m be A->bool;
  assume l INTER m = {x}                                                [H1];
  assume e IN l                                                         [H2];
  assume ~(e = x)                                                       [H3];
  thus ~(e IN m)

  proof
    assume e IN m;
    e IN l INTER m by H2, -, IN_INTER;
    e = x by -, H1, IN_SING;
    F by -, H3;
  qed by -`;;


let EquivIntersectionHelp_THM = thm `;
  let a x be A;
  let l m be A->bool;
  assume l INTER m = {x}                                                [H1];
  assume a IN m DELETE x                                                [H2];
  thus ~(a IN l) 

  proof
    a IN m /\ ~(a = x)                          [X1] by H2, IN_DELETE;
    qed by -, H1, H2, IntersectionSingletonOneNotOther_THM`;;


let B4'_THM = thm `;
  let l be line;
  let A B C be point;
  assume ~ Collinear(A, B, C) /\ ~(A IN l) /\ ~(B IN l) /\ ~(C IN l)    [H1];
  assume A,B same_side l /\ B,C same_side l                             [H2];
  thus A,C same_side l

  proof 
   ~(?Y. Y IN l /\ Between(A, Y, B)) /\ 
   ~(?Y. Y IN l /\ Between(B, Y, C)) ==> 
   ~(?X. X IN l /\ Between(A, X, C))  by H1, B4;
  qed by -, H1, H2, same_side_DEF`;;


let BetweenLinear_THM = thm `;
  let A C X be point;
  let m be line;
  assume A IN m /\ C IN m                                               [H1];
  assume Between(A, X, C)                                               [H2];
  thus X IN m

  proof
    ~(A = C) /\ Collinear(A, X, C)              [X1] by H2, B1;
    consider l such that
    A IN l /\ X IN l /\ C IN l                  [X2] by -, Collinear_DEF;
    l = m by X1, -, H2, H1, I1;
  qed by -, X2`;;

 
let CollinearLinear_THM = thm `;
  let A C X be point;
  let m be line;
  assume A IN m /\ C IN m                                               [H1];
  assume Collinear(A, X, C)                                             [H2];
  assume ~(A = C)                                                       [H3];
  thus X IN m

  proof
    consider l such that 
    A IN l /\ X IN l /\ C IN l                  [X1] by H2, Collinear_DEF;
    l = m by H3, -, H1, I1;
  qed by -, X1`;;
 

let Line01infinity_THM = thm `;
  let X be point;
  let l m be line;
  assume ~(l = m)                                                       [H1];
  assume X IN l /\ X IN m                                               [H2];
  thus l INTER m = {X}

  proof
    (l INTER m = {X}) \/ ~(l INTER m = {X});
    assume ~(l INTER m = {X})                                           [H3];
    X IN l INTER m by H2, IN_INTER;
    consider A such that
    A IN l INTER m  /\ ~(A = X)         [X1] by -, H3, BiggerThanSingleton_THM;
    A IN l /\ X IN l /\ A IN m /\ X IN m by -, H2, IN_INTER;
    l = m by -, X1, I1;
    F by -, H1;
  qed by -`;;
 

let EquivIntersection_THM = thm `;
  let A B X be point;
  let l m be line;
  assume l INTER m = {X}                                                [H1];
  assume A IN m DELETE X /\ B IN m DELETE X                             [H2];
  assume ~ Between(A, X, B)                                             [H3];
  thus  ~(A IN l) /\ ~(B IN l) /\ A,B same_side l

  proof
    A IN m /\ ~(A = X)                          [X1] by H2, IN_DELETE;
    B IN m /\ ~(B = X)                          [X2] by H2, IN_DELETE;
    ~(A IN l) /\ ~(B IN l) [X3] by H1, H2, EquivIntersectionHelp_THM;
    A,B same_side l                                                     [X4]
    proof
      assume ~(A,B same_side l);
      consider G such that 
      (G IN l) /\ Between(A, G, B)              [X5] by -, same_side_DEF;
      
       ~(A = B) /\ Collinear(A, G, B)           [X6] by -, B1;
      consider k such that 
      A IN k /\ G IN k /\ B IN k                [X7] by -, Collinear_DEF;
      k = m by -, X1, X2, X6, I1;
      G IN l INTER m by -, X5, X7, IN_INTER;
      G = X by -, H1, IN_SING;
      Between(A, X, B) by -, X5;
      F by -, H3;
    qed by -;
  qed by X3, X4`;;


let SameSideReflexiveRelation_THM = thm `;
  thus Reflexive_Property

  proof
    !l:line A:point. A,A same_side l
    proof
      let l be line;
      let A be point;
       ~(?X. (X IN l) /\ Between(A, X, A)) by B1;
    qed by -, same_side_DEF;
  qed by -, Reflexive_relation_DEF`;;


let SameSideSymmetricRelation_THM = thm `;
  thus Symmetric_Property

  proof
    !l:line A:point B:point. ~(A IN l) /\ ~(B IN l) ==> 
    A,B same_side l ==> B,A same_side l
    proof
      let l be line;
      let A B be point;
      assume A,B same_side l                                            [H1];
      assume ~(A IN l) /\ ~(B IN l);
      ~(?X. (X IN l) /\ Between(A, X, B)) by H1, same_side_DEF;
      ~(?X. (X IN l) /\ Between(B, X, A)) by -, B1;
    qed by -, same_side_DEF;
  qed by -, Symmetric_relation_DEF`;;


let SameSideTransitiveRelation_THM = thm `;
  thus Transitive_Property

  proof
    !l:line A:point B:point C:point. 
    ~(A IN l) /\ ~(B IN l) /\ ~(C IN l) ==> 
    A,B same_side l /\ B,C same_side l ==> A,C same_side l 
    proof
      let l be line;
      let A B C be point;
      assume ~(A IN l) /\ ~(B IN l) /\ ~(C IN l)                        [H0];
      assume A,B same_side l                                            [H1];
      assume B,C same_side l                                            [H2];
      A,C same_side l
      proof
        ~ Collinear(A, B, C) \/ Collinear(A, B, C);
        cases by -;
        suppose ~ Collinear(A, B, C);
        qed by -, H0, H1, H2, B4'_THM;
        suppose Collinear(A, B, C)                                      [Coll];
          cases;
          suppose A = B \/ A = C \/ B = C;
          qed by -, H2, H0, SameSideReflexiveRelation_THM, 
                            Reflexive_relation_DEF, H1;
          suppose ~(A = B) /\ ~(A = C) /\ ~(B = C)      [Distinct];
            consider m such that 
            A IN m /\ C IN m                            [W1] by Distinct, I1;
            ~(l = m)                                    [W2] by W1, H0;
            cases;
            suppose l INTER m = {}                      [Disjoint];
              !X. Between(A, X, C) ==> ~(X IN l)
              proof
                let X be point;
                assume Between(A, X, C);
                X IN m by -, W1, BetweenLinear_THM;
                ~(X IN l) by -, Disjoint, DisjointOneNotOther_THM; 
              qed by -;
            qed by -, same_side_DEF;
            suppose ~(l INTER m = {})                   [NotDisjoint];
              consider X such that
              X IN l INTER m by NotDisjoint, MEMBER_NOT_EMPTY;
              X IN l /\ X IN m                          [U1] by -, IN_INTER;
              l INTER m = {X}           [U2] by W2, -, Line01infinity_THM;
              consider E such that 
              E IN l /\ ~(E = X)                        [U3] by U1, I2;
              ~(E IN m) [U4] by U2, U3, IntersectionSingletonOneNotOther_THM;
              ~(E = B) by U3, H0;
              consider B' such that 
              Between(E, B, B') by -, B2;
              Between(B', B, E)                         [U5] by -, B1;
              ~(B' = E) /\ ~(B = E) /\ ~(B' = B) /\ Collinear(B', B, E) [U6] by 
-, B1;
              consider n such that 
              E IN n /\ B' IN n                         [U7] by -, I1;
              B IN n                    [U8] by U7, U5, BetweenLinear_THM;
              ~(l = n)                  [U9] by H0, -;
              l INTER n = {E} [U10] by U9, U7, U3, Line01infinity_THM;
              ~(B' IN l) [U11] by -, U7, U6, 
IntersectionSingletonOneNotOther_THM;
              ~ Between(B, E, B')        [U12] by U6, U5, B3;
              B' IN n DELETE E /\ B IN n DELETE E by U7, U8, U6, IN_DELETE;
              B, B' same_side l [U13] by U10, -, U12, EquivIntersection_THM;
              ~(m = n)                   [U14] by  U7, U4;
              B IN m by W1, Coll, Distinct, CollinearLinear_THM;
              m INTER n = {B} [U15]  by -, U8, U14, Line01infinity_THM;
              ~(A IN n) [U16] by -, W1, Distinct, 
IntersectionSingletonOneNotOther_THM;
              ~ Collinear(A, B, B')
              proof
                assume Collinear(A, B, B');
                consider k such that 
                A IN k /\ B IN k /\ B' IN k             [V1] by -, 
Collinear_DEF;
                k = n by U6, -, U8, U7, I1;
                F by -, U16, V1;
              qed by -;
              A,B' same_side l          [U17] by -, H0, U11, H1, U13, B4'_THM;
              ~(C IN n)                 [U18] by U15, W1, Distinct, 
              IntersectionSingletonOneNotOther_THM;
              C,B same_side l [U19] by H0, H2, SameSideSymmetricRelation_THM, 
              Symmetric_relation_DEF;
              ~ Collinear(C, B, B')
              proof
                assume Collinear(C, B, B');
                consider k such that 
                C IN k /\ B IN k /\ B' IN k             [V2] by -, 
Collinear_DEF;
                k = n by U6, -, U8, U7, I1;
                F by -, U18, V2;
              qed by -;
              C,B' same_side l by -, H0, U11, U19, U13, B4'_THM;
              B',C same_side l [U20] by H0, U11, -, 
SameSideSymmetricRelation_THM, 
              Symmetric_relation_DEF;
              ~(B' IN m) [U21] by U15, U7, U6, 
IntersectionSingletonOneNotOther_THM;

              ~ Collinear(A, B', C)
              proof
                assume Collinear(A, B', C);
                consider k such that 
                A IN k /\ B' IN k /\ C IN k             [V3] by -, 
Collinear_DEF;
                k = m by Distinct, W1, -, I1;
                F by -, U21, V3;
              qed by -;
              A, C same_side l by -, H0, U11, U17, U20, B4'_THM;
            qed by -;
          end;
        end;
      end;
    qed by -;
  qed by -, Transitive_relation_DEF`;;


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


let OnePointImpliesAnother_THM = thm `;
  let P be point; 
  thus ?Q:point. ~(Q = P)

  proof
    consider A B C such that 
    ~(A = B) /\ ~(A = C) /\ ~(B = C) /\ ~ Collinear(A, B, C) [X1] by I3;
    cases;
    suppose B = P;
      ~(A = B) by -, X1;
    qed by -;
    suppose ~(B = P);
    qed by -;
  end`;;


let NonCollinearImpliesDistinct_THM = thm `;
  let A B C be point;
  assume ~ Collinear(A, B, C)                                           [H1];
  thus ~(A = B) /\ ~(A = C) /\ ~(B = C)

  proof
    cases;
    suppose A = B /\ B = C                                              [C1];
      consider Q such that ~(Q = A) by OnePointImpliesAnother_THM;
      consider l such that A IN l /\ Q IN l by -, I1;
      Collinear(A, B, C) by -, C1, Collinear_DEF;
    qed by -, H1;
    suppose ~(A = B) /\ B = C                                           [C2];
      consider l such that A IN l /\ B IN l by -, I1;
      Collinear(A, B, C) by -, C2, Collinear_DEF;
    qed by -, H1;
    suppose ~(B = C)                                                    [C3];
      consider l such that B IN l /\ C IN l [X1] by C3, I1;
      ~(A = B)                                          [U] 
      proof
        assume A = B;
        Collinear(A, B, C) by -, X1, Collinear_DEF;
      qed by -, H1; 
      ~(A = C)                                          [V]
      proof
        assume A = C;
        Collinear(A, B, C) by -, X1, Collinear_DEF;
      qed by -, H1; 
    qed by U, V, C3;          
  end`;;


let ConverseCrossbar_THM = thm `;
  let O A B G be point;
  assume ~ Collinear(A, O, B)                   [H1];
  assume Between(A, G, B)                       [H2];
  thus int_angle(G, A, O, B)

  proof
    ~(A = O) /\ ~(A = B) /\ ~(O = B) [Distinct] by H1, 
                                                 
NonCollinearImpliesDistinct_THM;
    consider a such that O IN a /\ A IN a                         [aOA] by -, 
I1;
    consider b such that O IN b /\ B IN b                  [bOB] by Distinct, 
I1;
    consider l such that A IN l /\ B IN l                  [lAB] by Distinct, 
I1;
     ~(B IN a) by H1, aOA, Collinear_DEF;
    ~(a = l) by -, lAB;
    a INTER l = {A}                     [alA] by -, aOA, lAB, 
Line01infinity_THM;
    ~(A = G) /\ ~(A = B) /\ ~(G = B) /\  Between(B, G, A) /\ Collinear(A, G, B) 
                                                                  [X1] by H2, 
B1;
    ~ Between(G, A, B)                                 [notGAB] by -, H2, B3, 
B1;
    G IN l                                  [Ginl] by lAB, H2, 
BetweenLinear_THM;
   ~(G IN a)    [notGina] by alA, Ginl, X1, 
IntersectionSingletonOneNotOther_THM;
    G IN l DELETE A /\ B IN l DELETE A by Ginl, lAB, X1, IN_DELETE;
    G,B same_side a           [Gsim_aB] by alA, -, notGAB, 
EquivIntersection_THM;
    :: same argument shows   G,A same_side b
    ~(A IN b) by H1, bOB, Collinear_DEF;
    ~(b = l) by -, lAB;
    b INTER l = {B}                     [blB] by -, bOB, lAB, 
Line01infinity_THM;
    ~ Between(G, B, A)                  [notGBA] by H2, B1, B3;   
    ~(G IN b)   [notGinb] by blB, Ginl, X1, 
IntersectionSingletonOneNotOther_THM;
    G IN l DELETE B /\ A IN l DELETE B by Ginl, lAB, X1, IN_DELETE;
    G,A same_side b            [Gsim_bA]by blB, -, notGBA, 
EquivIntersection_THM;
  qed by H1, aOA, bOB, notGina, notGinb, Gsim_aB, Gsim_bA, InteriorAngle_DEF`;;


let WholeRayInterior_THM = thm `;
  let A O B X be point;
  assume ~ Collinear(A, O, B)                                           [H1];
  assume int_angle(X, A, O, B)                                          [H2];
  let P be point;
  assume ray(P, O, X)                                                   [H3];
  assume ~(P = O)                                                       [H4];
  thus int_angle(P, A, O, B) 

  proof
    ~(A = O) /\ ~(A = B) /\ ~(O = B) [Distinct] by H1, 
NonCollinearImpliesDistinct_THM;
    consider a such that O IN a /\ A IN a       [a_OA] by Distinct, I1;
    consider b such that O IN b /\ B IN b       [b_OB] by Distinct, I1;
    consider x such that O IN x /\ X IN x       [x_OX] by Distinct, I1;
    consider a' b' such that O IN a' /\ A IN a' /\ O IN b' /\ B IN b' /\
    ~(X IN a') /\ ~(X IN b') /\ X,B same_side a' /\ X,A same_side b' [Xint'] by 
H2, InteriorAngle_DEF;
    a' = a /\ b' = b by Distinct, -, a_OA, b_OB, I1;
    ~(X IN a) /\ ~(X IN b) /\ X,B same_side a /\ X,A same_side b [XintAOB] by 
-, Xint';
    ~(O = X) /\ Collinear(O, X, P) /\ ~ Between(P, O, X) [XrayPOX] by H3, 
Ray_DEF;
    P IN x       [Pin_x] by x_OX, XrayPOX, Collinear_DEF, CollinearLinear_THM;
    P IN x DELETE O                         [Pin_x_O] by Pin_x, H4, IN_DELETE;
    X IN x DELETE O                    [Xin_x_O] by x_OX, XrayPOX,  IN_DELETE;
    ~(x = a) /\ ~(x = b)               [x_not_ab] by XintAOB, x_OX;
    a INTER x = {O} /\ b INTER x = {O} [axb_intO] by x_not_ab, x_OX, a_OA, 
b_OB, Line01infinity_THM;
    ~(P IN a) /\ P,X same_side a [Psim_aX] by axb_intO, Pin_x_O, Xin_x_O, 
XrayPOX, EquivIntersection_THM;
    ~(P IN b) /\ P,X same_side b [Psim_bX] by axb_intO, Pin_x_O, Xin_x_O, 
XrayPOX, EquivIntersection_THM;
    P,B same_side a  /\ P,A same_side b by Psim_aX, Psim_bX, XintAOB, a_OA, 
b_OB, H1, Collinear_DEF, SameSideTransitiveRelation_THM, 
Transitive_relation_DEF;
    int_angle(P, A, O, B) by H1, a_OA, b_OB, Psim_aX, Psim_bX, -, 
InteriorAngle_DEF;
  qed by -`;;



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