<span class="identifier">hold</span><span class="special">[]</span></code>
            directive helps managing attributes, mainly for alternative parsers.
            It instantiates a new attribute instance for the embedded parser. The
            value of that attribute instance is copied to the outer attribute if
            the embedded parser succeeds and it is discarded otherwise. Alternative
            parsers normally do not rollback changes made to the outer attribute
            by an failed alt