code: public void addExtension(String name, Object object)
        throws SourceMapParseException{
      if (!name.startsWith("x_")){
        throw new SourceMapParseException("Extension '" + name +
                                          "' must start with 'x_'");
      }
      this.extensions.put(name, object);
    }
docs: Adds field extensions to the json source map. The value is allowed to be
  any value accepted by json, eg. string, JsonObject, JsonArray, etc.
  
  Extensions must follow the format x_orgranization_field (based on V3
  proposal), otherwise a {@code SourceMapParseExtension} will be thrown.
  
  @param name The name of the extension with format organization_field
  @param object The value of the extension as a valid json value
  @throws SourceMapParseException  if extension name is malformed