theorem Th79:
  A out2 a,b & A out2 b,c implies A out2 a,c
  proof
    assume that
A1: A out2 a,b and
A2: A out2 b,c;
    consider x be POINT of S such that
A3: between2 a,A,x and
A4: between2 b,A,x by A1;
    consider y be POINT of S such that
A5: between2 b,A,y and
A6: between2 c,A,y by A2;
    between2 x,A,b & between2 y,A,b by A4,A5,GTARSKI3:14;
    then between2 y,A,c & A out2 y,x by A6,GTARSKI3:14;
    then between2 x,A,c by Th74;
    then between2 c,A,x by GTARSKI3:14;
    hence thesis by A3;
  end;
