ssion <code>exp</code> evaluates to <code>bool_value</code> when <code>var</code> is set to true. Return None if it has free variables that stop us from evaluating it, or is not a boolean expression.</p></dd></dl><dl><dt class="spec value" id="val-ignore_cast"><a href="#val-ignore_cast" class="anchor"></a><code><span class="keyword">val</span> ignore_cast : <a href="index.html#type-t">t</a> <span>&#45;&gt;</span> <a href="index.html#type-t">t</a></code></dt><dt class="spec value" id="val-access_expr_of_exp"