docs: public void addForwardedField(int sourceField, int targetField) {
  		if(isTargetFieldPresent(targetField)) {
  			throw new InvalidSemanticAnnotationException("Target field "+targetField+" was added twice.");
  		}
  
  		FieldSet targetFields = fieldMapping.get(sourceField);
  		if (targetFields != null) {
  			fieldMapping.put(sourceField, targetFields.addField(targetField));
  		} else {
  			fieldMapping.put(sourceField, new FieldSet(targetField));
  		}
  	}