code: protected AstNode parseCustomStatement( DdlTokenStream tokens,
                                              AstNode parentNode ) throws ParsingException {
          assert tokens != null;
          assert parentNode != null;
  
          // DEFAULT DOES NOTHING
          // Subclasses can implement additional parsing
  
          return null;
      }
docs: Catch-all method to parse unknown (not registered or handled by sub-classes) statements.
  
  @param tokens the {@link DdlTokenStream} representing the tokenized DDL content; may not be null
  @param parentNode the parent {@link AstNode} node; may not be null
  @return the {@link AstNode}
  @throws ParsingException