docs: def source_get(method_name):
      """
      Creates a getter that will drop the current value,
      and call the source's method with specified name
      using the context's key as first argument.
      @param method_name: the name of a method belonging to the source.
      @type method_name: str
      """
  
      def source_get(_value, context, **_params):
          method = getattr(context["model"].source, method_name)
          return _get(method, context["key"], (), {})
  
      return source_get