I had two problems when was trying to create an obligation dynamically and when was trying to select a value from a bag using an expression as an index. Please let me know if it's possible at all. If not, please consider these features for future implementations.

I needed to generate a "show-message" obligation that would explain a reason of "deny" to an authorization service consumer. The message depends on rules and is calculated dynamically.

However, I can only return <AttributeValue> that is interpreted literally by PDP. The wanted feature is to use an arbitrary "Expression" instead of <AttributeValue>, e.g. to use a variable that has been evaluated before.

The second problem: I could have all messages enlisted in a string bag. The wanted feature is to be able to create an expression and use it as an index to select a string from a bag.

Please let me know if there is a way of doing that in the current implementation.