Custom item to increase memorized spells

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I'm trying to create a custom item for our campaign that allows a spellcaster to memorize one additional spell.

I assume I need to add code to the custom item's Eval Script, similar to:

field[????].value += 1

The problem I have is that I can't seem to locate tag that stores the number of memorized spells. I've searched the forums, google, and Hero Lab developer resources with no luck.

Any help would be highly appreciated!