code: public String mapStylesheetListNr(String listNr) {
          if(this.importStylesheetListMapping.containsKey(listNr)) {
              return (String) this.importStylesheetListMapping.get(listNr);
          } else {
              return "0";
          }
      }
docs: Performs the mapping from the original stylesheet number to the actual
  stylesheet number in the resulting RTF document. If the stylesheet number was not
  seen during import (thus no mapping) then 0 is returned, guaranteeing
  that the stylesheet number is always valid.