Form using Substitution Variable with List of Members

Summary:

Want to use a subvar with a list of members for a form.

Content (required):

We have a scenario for which users plan different months in different years. I can make a subvar with a list of members (eg. Mar,Jul,Sep,Dec). But forms do not accept this (unlike say Jan:Mar, which works fine in a subvar). When I try, it returns the error: "A substitution or user variable required for this calculation is undefined. Variable: xtest" I have tried double quotes around the subvar and around each member in the list and have also tried wrapping the subvar in a function (like ILvl0Descendant("&xtest)) without luck. Has anyone successfully done this?