fill (0,3)--(9,3)--(9,6)--(6,6)--(6,9)--
  (3,9)--(3,0)--(6,0)--(6,6)--(0,6)--\cycle;
unfill (0,3)--(9,3)--(9,6)--(0,6)--(0,3)--
 (3,3)--(3,0)--(6,0)--(6,9)--(3,9)--
  (3,3)--cycle;
tracingall;showall currentpicture;