<arg>id</arg> is an arbitrary string assigned by the user. <arg>id</arg> must be unique per application session per script and may contain alphanumeric characters and undescrore.

<arg>fn</arg> is the file name of the script, which is a path. Librnd does not do any search, it only opens the path as specified.

<arg>lang</arg> is the scripting language, as in fungw plugin name. When not specified, the code makes a guess (based on the file name).