"anchor"></a><code><span class="keyword">val</span> diff : <span><span>(<a href="index.html#type-t">t</a> <span>&#45;&gt;</span> <a href="index.html#type-t">t</a> <span>&#45;&gt;</span> <span><a href="index.html#type-t">t</a> list</span>)</span> option</span></code></dt><dd><p>substracts the second abstract element from the first (difference operator) if an exact operator can not be defined (None), the solver doesn't use the pruning features. precondition: the two abstract elements must be defined onto the 