span>(<span class="type-var">'a</span> <span>&#45;&gt;</span> <span class="type-var">'a</span> <span>&#45;&gt;</span> int)</span></span> <span>&#45;&gt;</span> <span><span class="type-var">'a</span> option</span></code></dt><dt class="spec value" id="val-invariant"><a href="#val-invariant" class="anchor"></a><code><span class="keyword">val</span> invariant : <span><span class="type-var">'a</span> Base__.Invariant_intf.inv</span> <span>&#45;&gt;</span> <span><span><span class="type-var">'a</span> <a href="in