theorem Th26:
  A,B,C is_a_triangle implies angle(B,A,C) - angle(C,B,A) <> -PI
  proof
    assume
A1: A,B,C is_a_triangle;
    then B,A,C is_a_triangle by MENELAUS:15;
    then
A2: B,A,C are_mutually_distinct by EUCLID_6:20;
    assume
A3: angle(B,A,C) - angle(C,B,A) = -PI;
    per cases by EUCLID11:3;
    suppose 0 <= angle(B,A,C) < PI;
      then
A4:   0 < angle(B,A,C) < PI by A1,EUCLID10:30;
      then 0 + PI < angle(B,A,C) + PI by XREAL_1:8;
      hence contradiction by A3,A4,A2,EUCLID11:5;
    end;
    suppose angle(B,A,C) = PI;
      hence contradiction by A3,EUCLID11:2;
    end;
    suppose PI < angle(B,A,C) < 2 * PI;
      then PI + PI < angle(B,A,C) + PI by XREAL_1:8;
      hence contradiction by A3,EUCLID11:2;
    end;
  end;
