   namespace a = "http://relaxng.org/ns/compatibility/annotations/1.0"

   # xml2rfc <referencegroup> grammar

   referencegroup =
     element referencegroup {
       attribute xml:base { text }?,
       attribute xml:lang { text }?,
       attribute anchor { xsd:ID },
       attribute derivedAnchor { text }?,
       attribute target { text }?,
       reference+
     }
   include "reference.rnc"

   start |= referencegroup
