theorem Th5:
  a,b // c,d & a,b // c,d9 implies d = d9
proof
  assume a,b // c,d & a,b // c,d9;
  then c,d // a,b & c,d9 // a,b by Th3;
  then c,d // c,d9 by Def1;
  hence thesis by Th4;
end;
