References the value of a variable. The variable can either be an external variable supported by the host application or an internal variable defined with <defvar>.
<ref>waveset.role</ref>
<defvar name=’milk’><s>milkvalue</s></defvar> <defvar name=’shake’><s>milk</s></defvar> <ref><ref>shake</ref>
evaluates to <s>milkshake</s>