A PropertyReadOnly exception indicates that an attempt was made to override
or overwrite a property that was set as read_only. For example, the following
will throw a PropertyReadOnly exception:
userWorkArea.begin("parent");
userWorkArea.set("key", "value", PropertyModeType.read_only);
userWorkArea.set("key", "value");
Likewise, the following sequence of operations is not permitted:
userWorkArea.begin("parent");
userWorkArea.set("key", "value", PropertyModeType.read_only);
userWorkArea.begin("child");
userWorkArea.set("key", "value");