* when the parameter is not a <code>VariableField</code>
* instance
*/
public void addVariableField(VariableField field) {
if (!(field instanceof VariableField))
throw new IllegalAddException("Expected VariableField instance");
String tag = field.getTag();
if (Verifier.isControlNumberField(tag)) {
if (Verifier.hasControlNumberField(controlFields))
controlFields.set(0, field);