public function apply_modifier( $key, $value ) {
		$call_args = func_get_args();

		$application = Tribe__Utils__Array::get( $this->schema, $key, null );

		/**
		 * Return primitives, including `null`, as they are.
		 */
		if ( ! is_callable( $application ) ) {
			return $application;
		}

		/**
		 * Allow for callbacks to fire immediately and return more complex values.
		 * This also means that callbacks meant to run on the next step, the one
		 * where args are applied, will need to be "wrapped" in callbacks themselves.
		 * The `$key` is removed from the args to get the value first and avoid
		 * unused args.
		 */
		$args_without_key = array_splice( $call_args, 1 );

		return call_user_func_array( $application, $args_without_key );
	}