injectIdentifier(doc, section) {
    let ident = this.book.packaging.metadata.identifier;
    let meta = doc.createElement("meta");
    meta.setAttribute("name", "dc.relation.ispartof");

    if (ident) {
      meta.setAttribute("content", ident);
    }

    doc.getElementsByTagName("head")[0].appendChild(meta);
  }
Hook to handle the document identifier before
a Section is serialized
@param  {document} doc
@param  {Section} section
@private