theorem
  Mid a,p,a9 & Mid b,q,b9 & MDist p,q implies a,b // b9,a9
proof
  assume that
A1: Mid a,p,a9 and
A2: Mid b,q,b9 and
A3: MDist p,q;
  Mid a,q,a9 by A1,A3,Th23;
  hence thesis by A2,Th25;
end;
