code: public Map<String, String> getCodeToPropIdMap() {
          Map<String, String> result
                  = new HashMap<>(this.codeToPropIdMap);
          return result;
      }
docs: Returns a one-to-one mapping from code to proposition id. If
  <code>null</code> or a mapping for a code is not defined, it is assumed
  that the code in the database is the same as the proposition id.
  
  @return a {@link Map<String,String>}. Guaranteed not <code>null</code>.