String valueIsNotStr = element.getAttribute(VALUE_IS_NOT, null);
Value valueIs = null;
Value valueIsNot = null;
if (guardPropName != null) {
Property guardProp = script.getProperty(guardPropName);
if (guardProp == null) {
error("A guard property '" + guardPropName + "' not declared", element);
}
if (valueIsStr != null && valueIsNotStr != null) {
error("The '" + VALUE_IS + "' and '" + VALUE_IS_NOT +
"' attributes cannot be used together", element);
}
PropertyType type = guardProp.getType();
if (valueIsStr != null) {
valueIs = type.fromValue(valueIsStr);
if (valueIs == null) {
error("The string '" + valueIsStr + "' is not a valid " + type.getTypeName() +
" instance", element);