code: public void merge( Context context, Writer writer)
          throws ResourceNotFoundException, ParseErrorException, MethodInvocationException
      {
          merge(context, writer, null);
      }
docs: The AST node structure is merged with the
  context to produce the final output.
  
  @param context Conext with data elements accessed by template
  @param writer output writer for rendered template
  @throws ResourceNotFoundException if template not found
  from any available source.
  @throws ParseErrorException if template cannot be parsed due
  to syntax (or other) error.
  @throws MethodInvocationException When a method on a referenced object in the context could not invoked.