theorem Th14:
  <*x/"y*> ==>. (x/"z)/"(y/"z)
proof
A1: <*x/"y*>^<*y*> ==>. x by Th12;
  <*z*> ==>. z by Def18;
  then <*x/"y*>^<*y/"z*>^<*z*> ==>. x by A1,Lm2;
  then <*x/"y*>^<*y/"z*> ==>. x/"z by Def18;
  hence thesis by Def18;
end;
