That's not that easy. The last I head (unless anything changed with recent hotfixes), SAS RTDM up to 6.5 did not have an option to create a calculated item of a List type. Regular scalar types - yes, data grid - yes, but not lists... That's strange and illogical but that was the case (unless I missed something).
You may end up needing to create a SAS Process (a small piece of custom ds2 code) to achieve the task.
Best regards, Dmitriy.
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Dmitriy Alergant, Tier One Analytics