The root element identifying a RelaxNG grammar.
<grammar xmlns="http://relaxng.org/ns/structure/1.0">
<start>
...
</start>
<define name="def1">
...
</define>
<define name="def2">
...
</define>
</grammar>
A grammar element has one 'start' child-element.
A grammar element can have multiple 'define' child-elements.
RelaxNG Namespace: http://relaxng.org/ns/structure/1.0