Return the textual content of the INDEXth argument.
INDEX is based from the current processing position. If INDEX is
positive, values returned are closer to the command argument; if
negative, they are closer to the last argument. If the INDEX is
outside of the argument list, nil is returned. The default value for
INDEX is 0, meaning the current argument being examined.
The special indices `first' and `last' may be used to access those
parts of the list.
The OFFSET argument is added to/taken away from the index that will be
used. This is really only useful with `first' and `last', for
accessing absolute argument positions. (fn &optional INDEX OFFSET)