 is the concatenation of all elements in <code>xs</code> into a single value, derived by use of <code>op</code>.</p><p>This is equivalent to <code>List.fold_right op xs unit</code>.</p><p>The initial <i>m</i> marks this as monoidal concatenation rather than the <a href="index.html#val-concat"><code>concat</code></a> function inherited from the underlying <a href="../../../Semigroup/module-type-S/index.html"><span>Semigroup</span></a>. Since <a href="index.html#val-mconcat"><code>mconcat</code></a> can use t