<tree>

<tree> encodes a tree, which is made up of a root, internal nodes, leaves, and arcs from root to leaves. 19.2 Trees
組件nets — 19 Graphs, Networks, and Trees
屬性
aritygives the maximum number of children of the root and internal nodes of the tree.
狀態 非必備的
資料類型 
data.count
值 A nonnegative integer.
ord (ordered) indicates whether or not the tree is ordered, or if it is partially ordered.
狀態 必備的
合法的值是:
true
indicates that all of the branching nodes of the tree are ordered. [預設值]
partial
indicates that some of the branching nodes of the tree are ordered and some are unordered.
false
indicates that all of the branching nodes of the tree are unordered.
ordergives the order of the tree, i.e., the number of its nodes.
狀態 非必備的
資料類型 
data.count
值 A nonnegative integer.
註
The size of a tree is always one less than its order, hence there is no need for both a size and order attribute.
屬於forest model.divPart
可包含
core: label
nets: iNode leaf root
宣告
element tree
{
   att.global.attributes,
   attribute arity { data.count }?,
   attribute ord { "true" | "partial" | "false" },
   attribute order { data.count }?,
   ( label?, ( ( leaf | iNode )*, root, ( leaf | iNode )* ) )
}
例子
<tree
  n="ex2"
  arity="2"
  ord="partial"
  order="13">
 <root xml:id="G-div1" children="#G-plu1 #G-exp1" ord="true">
  <label>/</label>
 </root>
 <iNode
   xml:id="G-plu1"
   children="#G-exp2 #G-exp3"
   parent="#G-div1"
   ord="false">
  <label>+</label>
 </iNode>
 <iNode
   xml:id="G-exp1"
   children="#G-plu2 #G-num2.3"
   parent="#G-div1"
   ord="true">
  <label>**</label>
 </iNode>
 <iNode
   xml:id="G-exp2"
   children="#G-vara1 #G-num2.1"
   parent="#G-plu1"
   ord="true">
  <label>**</label>
 </iNode>
 <iNode
   xml:id="G-exp3"
   children="#G-varb1 #G-num2.2"
   parent="#G-plu1"
   ord="true">
  <label>**</label>
 </iNode>
 <iNode
   xml:id="G-plu2"
   children="#G-vara2 #G-varb2"
   parent="#G-exp1"
   ord="false">
  <label>+</label>
 </iNode>
 <leaf xml:id="G-vara1" parent="#G-exp2">
  <label>a</label>
 </leaf>
 <leaf xml:id="G-num2.1" parent="#G-exp2">
  <label>2</label>
 </leaf>
 <leaf xml:id="G-varb1" parent="#G-exp3">
  <label>b</label>
 </leaf>
 <leaf xml:id="G-num2.2" parent="#G-exp3">
  <label>2</label>
 </leaf>
 <leaf xml:id="G-vara2" parent="#G-plu2">
  <label>a</label>
 </leaf>
 <leaf xml:id="G-varb2" parent="#G-plu2">
  <label>b</label>
 </leaf>
 <leaf xml:id="G-num2.3" parent="#G-exp1">
  <label>2</label>
 </leaf>
</tree>
註
A root, and zero or more internal nodes and leaves, but if there is an internal node, there must also be at least one leaf.