ode></h3>
<p>
Returns a pattern equivalent to <em>!patt2 patt1</em>.
This pattern asserts that the input does not match
<code>patt2</code> and then matches <code>patt1</code>.
</p>

<p>
When succeeded,
this pattern produces all captures from <code>patt1</code>.
It never produces any capture from <code>patt2</code>
(as either <code>patt2</code> fails or
<code>patt1 - patt2</code> fails).
</p>

<p>
If both <code>patt1</code> and <code>patt2</code> are
character sets,
this operation is equivalent to set differ