code: public FormInputHandler checkTrafo(final ElementTrafo theCheckTrafo) {
  		Fields fields = new Fields(this);
  		fields.checkTrafo = theCheckTrafo;
  		return new FormInputHandler(fields);
  	}
docs: Creates a new {@link FormInputHandler} based on this {@link FormInputHandler} using the specified {@link ElementTrafo} for
  transforming the value before it is checked.
  
  @param theCheckTrafo
  the {@link ElementTrafo} to transform a value before it is checked when step mode is
  {@link StepMode#CHECK_VALUE}
  @return the new {@link FormInputHandler} instance