code: public MarcValueTransformers setMarcValueTransformer(String fieldKey, MarcValueTransformer transformer) {
          // split into  tag + indicator and subfield IDs
          int pos = fieldKey.lastIndexOf('$');
          String tagind = pos > 0 ? fieldKey.substring(0, pos) : fieldKey;
          String subs = pos > 0 ? fieldKey.substring(pos + 1) : "";
          this.marcValueTransformerMap.put(tagind, transformer);
          // subfield IDs
          this.subfieldMap.put(tagind, subs);
          return this;
      }
docs: Set MARC value transformer for transforming field values.
  @param fieldKey the MARC field key for the field to be transformed
  @param transformer the string transformer
  @return this handler