">IR.Ident.t</a> <span class="arrow">&#45;&gt;</span></span> <span><a href="../../IR/Exp/index.html#type-t">IR.Exp.t</a> <span class="arrow">&#45;&gt;</span></span> <span><a href="#type-subst">subst</a> option</span></span></code></div><div class="spec-doc"><p>Extend substitution and return <code>None</code> if not possible.</p></div></div><div class="odoc-spec"><div class="spec value" id="val-subst_free_vars" class="anchored"><a href="#val-subst_free_vars" class="anchor"></a><code><span><span class="keywor