9 Trees
  
  The  trees  are  used in ToDoLists. They are a technical feature, and fairly
  general, so they also can be used somewhere else.
  
  
  9.1 Trees
  
  9.1-1 IsTree
  
  IsTree( arg )  filter
  Returns:  true or false
  
  The  category  of  trees. A tree may have a content, a list of successors, a
  predecessor and it knows if it is a leave of a tree or not.
  
  9.1-2 Content
  
  Content( arg )  attribute
  Returns:  object
  
  The content of the tree. May be any object.
  
  9.1-3 ListOfSuccessors
  
  ListOfSuccessors( arg )  operation
  Returns:  a list of trees
  
  Returns the list of successors of a tree.
  
  9.1-4 Predecessor
  
  Predecessor( arg )  operation
  Returns:  a tree or fail
  
  Returns the predecessor of a tree, or fail if there is none.
  
  9.1-5 ListOfSentinels
  
  ListOfSentinels( arg )  operation
  Returns:  a list
  
  Returns a list of leaves of the tree.
  
  9.1-6 RemoveHead
  
  RemoveHead( arg )  operation
  Returns:  a tree
  
  Returns  the  first  successor of the tree, and adds all other successors of
  the tree to the tree that is returned. If the tree is a leave, it returns an
  empty tree. If the tree is empty, it returns the tree itself.
  
  9.1-7 Tree
  
  Tree(  )  operation
  Returns:  a tree
  
  Returns an empty tree.
  
  9.1-8 Tree
  
  Tree( obj )  operation
  Returns:  a tree
  
  Returns a tree with argument obj.
  
  9.1-9 Add
  
  Add( tree, new_tree )  operation
  Returns:  nothing
  
  Adds the [list of] tree[s] new_tree as successor to the tree tree.
  
  9.1-10 ContentListFromSentinelToHead
  
  ContentListFromSentinelToHead( sent )  operation
  Returns:  a list
  
  Returns  a  list  of the contents of the trees from the leave sent up to the
  content of the head of the tree.
  
  9.1-11 PostOrder
  
  PostOrder( arg )  operation
  Returns:  a list
  
  Returns the contents of the nodes of the tree in post-order.