code: public String mapFontNr(String fontNr) {
          if(this.importFontMapping.containsKey(fontNr)) {
              return (String) this.importFontMapping.get(fontNr);
          } else {
              return "0";
          }
      }
docs: Performs the mapping from the original font number to the actual
  font number in the resulting RTF document. If the font number was not
  seen during import (thus no mapping) then 0 is returned, guaranteeing
  that the font number is always valid.
  
  @param fontNr The font number to map.
  @return The mapped font number.