def _post_order(node, stack=None):
 if (stack is None):
  stack = list()
 if (type(node) is list):
  if (len(node) > 3):
   _post_order(node[3], stack)
  if (len(node) > 4):
   _post_order(node[4], stack)
  stack.append(node[0])
 else:
  stack.append(node)
 return stack

def _post_order ( node , stack = None ) : if ( stack is None ) : stack = list ( ) if ( type ( node ) is list ) : if ( len ( node ) > 3 ) : _post_order ( node [ 3 ] , stack ) if ( len ( node ) > 4 ) : _post_order ( node [ 4 ] , stack ) stack . append ( node [ 0 ] ) else : stack . append ( node ) return stack
generate a stack from a portion of the tree .