/index.html#type-re">Tree.re</a> <span class="arrow">&#45;&gt;</span></span> <span>string list</span></span></code></div><div class="spec-doc"><p><code>get_alphabet r</code></p><ul class="at-tags"><li class="returns"><span class="at-tag">returns</span> <p>the alphabet of RE <code>r</code> as a list</p></li></ul></div></div><div class="odoc-spec"><div class="spec value anchored" id="val-is_nullable"><a href="#val-is_nullable" class="anchor"></a><code><span><span class="keyword">val</span> is_nullable : <span