a> <span class="arrow">&#45;&gt;</span></span> bool )</span> <span class="arrow">&#45;&gt;</span></span> <span><a href="#type-subst">subst</a> <span class="arrow">&#45;&gt;</span></span> <a href="#type-subst">subst</a></span></code></div><div class="spec-doc"><p><code>sub_filter filter sub</code> restricts the domain of <code>sub</code> to the identifiers satisfying <code>filter</code>.</p></div></div><div class="odoc-spec"><div class="spec value" id="val-sub_filter_pair" class="anchored"><a href="#val-sub_