code: function MarcControlField(tag, data) {
    if (!(this instanceof MarcControlField)) {
      return new MarcControlField(tag, data);
    }
  
    if (tag instanceof MarcControlField) {
      // Copy the source field.
      var srcField = tag;
      this.tag = srcField.tag;
      this.data = srcField.data;
    } else {
      // Initialize field.
      MarcVariableField.call(this, tag);
      this.data = data || '';
    }
  }
docs: /*
  Represents a control field in a MARC record.