code: public String mapListNr(String listNr) {
          if(this.importListMapping.containsKey(listNr)) {
              return (String) this.importListMapping.get(listNr);
          } else {
              return null;
          }
      }
docs: Performs the mapping from the original list number to the actual
  list number in the resulting RTF document. If the list number was not
  seen during import (thus no mapping) then null is returned. There is no
  guarantee of a valid list number.