code: function MarcSubfield(code, data) {
    if (!(this instanceof MarcSubfield)) {
      return new MarcSubfield(code, data);
    }
  
    if (code instanceof MarcSubfield) {
      // Copy the source subfield.
      var srcSubfield = code;
      this.code = srcSubfield.code;
  
      if (srcSubfield.data instanceof MarcControlField) {
        this.data = new MarcControlField(srcSubfield.data);
      } else if (srcSubfield.data instanceof MarcDataField) {
        this.data = new MarcDataField(srcSubfield.data);
      } else {
        this.data = srcSubfield.data;
      }
    } else {
      // Initialize subfield.
      this.code = code || '?';
      this.data = data || '';
    }
  }
docs: /*
  Represents a subfield in a MARC record.