code: function MarcDataField(tag, ind1, ind2, subfields) {
    if (!(this instanceof MarcDataField)) {
      return new MarcDataField(tag, ind1, ind2, subfields);
    }
  
    if (tag instanceof MarcDataField) {
      // Copy the source field.
      var srcField = tag;
      this.tag = srcField.tag;
      this.ind1 = srcField.ind1;
      this.ind2 = srcField.ind2;
      this.subfields = [];
  
      for (var i = 0; i < srcField.subfields.length; i++) {
        this.addSubfield(new MarcSubfield(srcField.subfields[i]));
      }
    } else {
      // Initialize field.
      MarcVariableField.call(this, tag);
      this.ind1 = ind1 || ' ';
      this.ind2 = ind2 || ' ';
      this.subfields = subfields || [];
    }
  }
docs: /*
  Represents a data field in a MARC record.