Define "participate in a tree" instead of just "participate".

This makes linking from other specs simpler, since they have to mention
trees in the human-readable text anyway.

dom.bs

@@ -234,8 +234,9 @@ A <dfn export id=concept-tree>tree</dfn> is a finite hierarchical tree structure
 traversal of a <a>tree</a>.
 <!-- https://en.wikipedia.org/wiki/Depth-first_search -->
 
-An object that <dfn export for=tree id=concept-tree-participate lt="participate">participates</dfn> in
-a <a>tree</a> has a
+An object that <dfn export id=concept-tree-participate
+lt="participate in a tree|participates in a tree">participates in
+a <a>tree</a></dfn> has a
 <dfn export for=tree id=concept-tree-parent>parent</dfn>, which is either another object
 or null, and an ordered list of zero or more
 <dfn export for=tree id=concept-tree-child lt="child|children">child</dfn> objects. An object <var>A</var> whose
@@ -244,7 +245,7 @@ or null, and an ordered list of zero or more
 
 <p>The <dfn export for=tree id=concept-tree-root>root</dfn> of an object is itself, if its
 <a>parent</a> is null, or else it is the <a for=tree>root</a> of its <a>parent</a>. The
-<a for=tree>root</a> of a <a>tree</a> is any object <a for=tree>participating</a> in that
+<a for=tree>root</a> of a <a>tree</a> is any object <a lt="participate in a tree">participating</a> in that
 <a>tree</a> whose <a for=tree>parent</a> is null.
 
 An object <var>A</var> is called a
@@ -577,8 +578,7 @@ if(obj.dispatchEvent(event)) {
 
 When an <a>event</a> is
 <a>dispatched</a> to an object that
-<a>participates</a> in a
-<a>tree</a> (e.g. an
+<a>participates in a tree</a> (e.g. an
 <a for="/">element</a>), it can reach
 <a>event listeners</a> on that object's
 <a>ancestors</a> too. First all object's
@@ -762,8 +762,8 @@ the following:
  <dt><dfn const for=Event>CAPTURING_PHASE</dfn> (numeric value 1)
  <dd>When an <a>event</a> is
  <a>dispatched</a> to an object that
- <a>participates</a> in a
- <a>tree</a> it will be in this phase before it
+ <a>participates in a tree</a>
+ it will be in this phase before it
  reaches its {{Event/target}} attribute value.
  <dt><dfn const for=Event>AT_TARGET</dfn> (numeric value 2)
  <dd>When an <a>event</a> is
@@ -772,8 +772,8 @@ the following:
  <dt><dfn const for=Event>BUBBLING_PHASE</dfn> (numeric value 3)
  <dd>When an <a>event</a> is
  <a>dispatched</a> to an object that
- <a>participates</a> in a
- <a>tree</a> it will be in this phase after it
+ <a>participates in a tree</a>
+ it will be in this phase after it
  reaches its {{Event/target}} attribute value.
 </dl>
 Initially the attribute must be initialized to
@@ -1392,7 +1392,7 @@ can be used to explore this matter in more detail.
 
 <p>{{Document}}, {{DocumentType}}, {{DocumentFragment}}, {{ShadowRoot}}, {{Element}}, {{Text}},
 {{ProcessingInstruction}}, and {{Comment}} objects (simply called
-<dfn export id=concept-node>nodes</dfn>) <a>participate</a> in a <a>tree</a>, simply named the
+<dfn export id=concept-node>nodes</dfn>) <a>participate in a tree</a>, simply named the
 <dfn export id=concept-node-tree>node tree</dfn>.
 
 <p>A <a>node tree</a> is constrained as follows, expressed as a relationship between the type of