code: public List<?> getDiscreteDomain(){
  		Field<?> field = getField();
  
  		if(field instanceof HasDiscreteDomain){
  			List<?> validValues = FieldUtil.getValidValues((Field & HasDiscreteDomain)field);
  
  			if(validValues != null && !validValues.isEmpty()){
  				return validValues;
  			}
  		}
  
  		return null;
  	}
docs: <p>
  Returns the domain of valid values for this categorical or ordinal field.
  If specified, then all input values that are contained in this list shall be considered valid, and all others invalid.
  In not specified, then all input values shall be considered valid.
  </p>
  
  <p>
  List elements are all valid values in PMML representation.
  For example, if the data type of this field is {@link DataType#INTEGER}, then all list elements shall be {@link Integer}.
  </p>
  
  @return A non-empty list, or <code>null</code>.
  
  @see #getDataType()
  @see #getOpType()
  
  @see TypeUtil#format(Object)
  @see TypeUtil#parse(DataType, String)
  @see TypeUtil#parseOrCast(DataType, Object)