List of <i>features</i>. Default is the empty list.<br/>
<i>Features</i> on a rule modify the <i>features</i> currently enabled on
the <a href="#package">package</a> level via the <i>features</i> attribute.<br/>
For example, if the features ['a', 'b'] are enabled on the package level,
and a rule <i>features</i> attribute contains ['-a', 'c'], the features
enabled for the rule will be 'b' and 'c'.
</p>