e> will take any foldable structure containing objects forming a Ring and reduce them using the Ring's binary operation. The initial state for folding is the identity of the Ring's operation. It is sometimes necessary to specify the Ring to use; this is possible by using <code>product&lt;R&gt;</code>. If no Ring is specified, the structure will use the Ring formed by the elements it contains (if it knows it), or <code><a class="el" href="structboost_1_1hana_1_1integral__constant__tag.html" title="Tag repres