I want to be able to check if a string doesn't exist in an array.
Basically, it's this except the opposite of 'contains':
(if: $inv contains "Phone")[[[Remove Phone]]]
//What I want it to do
(if: $inv does not contain "Phone")[[[Remove Phone]]]
You use the Boolean not operator to invert the Boolean value returned from the contains operator like so...
(set: $inv to (a: "Card", "Pen", "Keys"))
(if: not ($inv contains "Phone"))[You forgot your phone!]