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### 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.

 ‣ 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.

 ‣ Add( tree, new_tree ) ( operation )

Returns: nothing

Adds the [list of] tree[s] new_tree as successor to the tree tree.

 ‣ 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.

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