String expression = builder.toString();
// The expression returns string value or throws an exception.
// It uses occupiedPropertyCode and freePropertyCode strings as special return values.
// All other return values are treated as error message.
EvaluateCallback evaluateCallback = new EvaluateCallback() {
// A message that shows when preview encountered an exception. Since preview
// is optional, the exception is reported as warning.
private final String warningMessage =
Messages.LogicImpl_WARNING_SEEMS_A_PROBLEM;