How to configure payload attributes (using Expression Builder) in BPM Worklist

Summary:
The business is asking for a way to prohibit a specific user to approve for a specific rule only within the Document of Record Approval. Is there a way to configure (using the Expression Builder) to best achieve this?

So far the syntax I have used under Configuration > Prohibit User Self-Approval > Payload Attribute

#{(task:task/task:payload/ns4:getNewDoRByTransactionIdResponse/ns4:result/ns2:DocumentType=='Document Record Name')&&('userid_of_approver')}

Also is there a recent white paper or documentation on general syntax cases for the Expression Builder? Thank you in advance for your help on this!

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