start = library
library = element library { book+, author*, character* }
author =
  element author {
    attribute id { xsd:ID },
    element name { text },
    element nickName { text },
    element born { text },
    element dead { text }
  }
book =
  element book {
    id-attribute,
    isbn,
    title,
    element author-ref {
      attribute id { xsd:IDREF },
      empty
    }*,
    element character-ref {
      attribute id { xsd:IDREF },
      empty
    }*
  }
id-attribute = attribute id { xsd:ID }
character =
  element character { id-attribute, name, since, qualification }
isbn = element isbn { text }
name = element name { text }
nickName = element nickName { text }
qualification = element qualification { text }
since = element since { xsd:date }
title = element title { text }