def prefixes
      prefixes = []
      prefixes = parent.prefixes if parent
      prefixes |= attributes.prefixes
      return prefixes
    end
def prefixes
      prefixes = []
      prefixes = parent.prefixes if parent
      prefixes |= attributes.prefixes
      return prefixes
    end
once :whitespace, :raw, :ignore_whitespace_nodes

 Namespaces                                    

 Evaluates to an +Array+ containing the prefixes (names) of all defined
 namespaces at this context node.
  doc = Document.new("<a xmlns:x='1' xmlns:y='2'><b/><c xmlns:z='3'/></a>")
  doc.elements['//b'].prefixes # -> ['x', 'y']