code: public MarcField tail(MarcField marcField, String key, MarcField appendToThisField) {
          if (key == null) {
              return marcField;
          }
          MarcField newMarcField = appendToThisField;
          if (newMarcField == null) {
              newMarcField = get(key);
          }
          if (lastReceived != null) {
              String lastKey = getTransformKey(lastReceived);
              if (key.equals(lastKey)) {
                  repeatCounter++;
              } else {
                  repeatCounter = 0;
              }
          }
          lastReceived = marcField;
          MarcField.Builder builder = MarcField.builder();
          if (appendToThisField != null) {
              builder.marcField(appendToThisField);
          } else {
              builder.tag(newMarcField.getTag())
                      .value(marcField.getValue());
              if (ignoreIndicator) {
                  builder.indicator(marcField.getIndicator());
              } else {
                  builder.indicator(interpolate(marcField, newMarcField.getIndicator()));
              }
          }
          if (ignoreSubfieldIds) {
              // just copy subfields as they are
              for (MarcField.Subfield subfield : marcField.getSubfields()) {
                  builder.subfield(subfield.getId(), subfield.getValue());
              }
          } else {
              // get the correct MARC field to map subfield IDs
              MarcField marcField1 = get(key);
              Iterator<MarcField.Subfield> subfields = marcField.getSubfields().iterator();
              Iterator<MarcField.Subfield> newSubfields = marcField1.getSubfields().iterator();
              while (subfields.hasNext() && newSubfields.hasNext()) {
                  MarcField.Subfield subfield = subfields.next();
                  MarcField.Subfield newSubfield = newSubfields.next();
                  builder.subfield(newSubfield.getId(), subfield.getValue());
              }
          }
          lastBuilt = builder.build();
          return lastBuilt;
      }
docs: Tail (appending) mode.
  @param marcField MARC field
  @param key key for the MARC field
  @param appendToThisField the MARC field to append to
  @return transformed MARC field