Essbase calc - shorter way to write a script based on Prior Periods

Summary:

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I'm working on a rule that requires the cumulation of prior month values (tax in this case) to be calculated into the current forecast month (back-dated tax). For example, if I am doing my forecast for March, and closed months are Jan and Feb, If the accountant hasn't posted a tax for any of the closed months, I need the calculation to include that month's tax in the March forecast - referred to as 'back dated tax'. See example below:

Example 1: No tax posted in Jan or Feb

Example 2 - Actual tax posted in Jan so we can ignore it in the calculation.