The root element of a Relax NG grammar contained in a binding definition. Refer to the topic entitled Creating a binding definition for further information.
<rng:grammar>
<rng:start>
<rng:element name="cool:folding-item" bd:component="true"/>
</rng:start>
<rng:define name="elements">
<rng:element name="cool:summary"/>
<rng:element name="cool:details"/>
</rng:define>
</rng:grammar>
Please see the topic entitled Creating a reusable component for a concrete example.