anchor.diff:  xpath_node_set xml_node::select_nodes(const xpath_query& query) const;
   ----
   
  -If exception handling is not disabled, both functions throw [link xpath_exception] if the query returns a value with type other than node set.
  +If exception handling is not disabled, both functions throw <<xpath_exception,xpath_exception>> if the query returns a value with type other than node set.
   
   This is an example of selecting nodes using XPath expressions (link:samples/xpath_select.cpp[]):
   
  
positive.code: 
  NOTE: The variable set pointer is stored in the query object; you have to ensure that the lifetime of the set exceeds that of query object.
  
  [#xpath_variable_set]
  Variable sets correspond to `xpath_variable_set` type, which is essentially a variable container.
  
  [#xpath_variable_set::add]
  You can add new variables with the following function:
  
  [source]
  
negative.code: 
  The attribute is copied along with the name and value; the node is copied along with its type, name and value; additionally attribute list and all children are recursively cloned, resulting in the deep subtree clone. The prototype object can be a part of the same document, or a part of any other document.
  
  The failure conditions resemble those of `append_child`, `insert_child_before` and related functions, <<xml_node::append_child,consult their documentation for more information>>. There are additional caveats specific to cloning functions:
  
  * Cloning null handles results in operation failure;
  * Node cloning starts with insertion of the node of the same type as that of the prototype; for this reason, cloning functions can not be directly used to clone entire documents, since <<node_document,node_document>> is not a valid insertion type. The example below provides a workaround.
  * It is possible to copy a subtree as a child of some node inside this subtree, i.e. `node.append_copy(node.parent().parent());`. This is a valid operation, and it results in a clone of the subtree in the state before cloning started, i.e. no infinite recursion takes place.
  
  This is an example with one possible implementation of include tags in XML (link:samples/include.cpp[]). It illustrates node cloning and usage of other document modification functions:
  