: hglass 
  east 2* west + 2* south + 2* north + 2* center +
  { 0 1 1 1 0 0 0 0 
    0 0 0 1 0 0 0 0 
    0 0 0 0 0 1 0 0
    0 1 0 0 0 1 1 1 }
  >pln0
;

: rule
  hglass
  center >pln1
;