OldValue
The OldValue function is part of the Trigger class.
It returns the previous data value from ARDI, where 'previous' means the values in the Values array the last time your function was called.
These are in the order that you specified in the '@data' decorator for your function.
The function has an optional parameter of the index in the array you'd like to read. If you don't specify, it assumes you want the 0th index.
Note that if a previous event can't be found, these values might be None