Parser.XML.Tree: Declare low_clone() as optional.

Fixes issues assigning eg a SimpleElementNode object to
a variable declared as a SimpleNode in strict_types mode.

Fixes [PIKE-122].

.gitattributes

@@ -159,7 +159,6 @@ testfont binary
 /lib/modules/Parser.pmod/Python.pmod foreign_ident
 /lib/modules/Parser.pmod/RCS.pike foreign_ident
 /lib/modules/Parser.pmod/SGML.pike foreign_ident
-/lib/modules/Parser.pmod/XML.pmod/Tree.pmod foreign_ident
 /lib/modules/Parser.pmod/XML.pmod/Validating.pike foreign_ident
 /lib/modules/Parser.pmod/XML.pmod/testsuite.in foreign_ident
 /lib/modules/Parser.pmod/module.pmod foreign_ident

lib/modules/Parser.pmod/XML.pmod/Tree.pmod

 #pike __REAL_VERSION__
 
 /*
- * $Id: Tree.pmod,v 1.72 2008/06/28 16:36:55 nilsson Exp $
+ * $Id$
  *
  */
 
@@ -283,7 +283,7 @@ class AbstractSimpleNode {
   //! Returns an initialized copy of the node.
   //! @note
   //!   The returned node has no children.
-  AbstractSimpleNode low_clone() {
+  optional AbstractSimpleNode low_clone() {
     return AbstractSimpleNode();
   }
 
@@ -537,7 +537,7 @@ class AbstractNode {
   //! Returns an initialized copy of the node.
   //! @note
   //!   The returned node has no children, and no parent.
-  AbstractNode low_clone()
+  optional AbstractNode low_clone()
   {
     return AbstractNode();
   }
@@ -1309,7 +1309,7 @@ class SimpleNode
   inherit VirtualNode;
 
   // Needed for cross-overloading
-  SimpleNode low_clone()
+  optional SimpleNode low_clone()
   {
     return VirtualNode::low_clone();
   }
@@ -1323,7 +1323,7 @@ class Node
   inherit VirtualNode;
 
   // Needed for cross-overloading
-  Node low_clone()
+  optional Node low_clone()
   {
     return VirtualNode::low_clone();
   }