code: public UNode toDoc() {
          // The root node is a MAP whose name is the alias name. We set its tag name to
          // "alias" for XML.
          UNode statNode = UNode.createMapNode(m_aliasName, "alias");
          
          // Add the expression parameter always, marked as an attribute.
          statNode.addValueNode("expression", getExpression(), true);
          return statNode;
      }
docs: Serialize this alias definition into a {@link UNode} tree and return the root node.
  
  @return  The root node of a {@link UNode} tree representing this alias.