How do you increment a variable within an each function?

[gabereal]

Hello, 

I am trying to see if it's possible to increment a variable within an each function. 

#"Table With EventKey" = Table.AddColumn(#"OriginalData", "EventKeyTable", each let
        AllData = [Custom],
        Counter = 0,
        AddToCounter = () => Counter = Counter +1,
        PrevRowComparison = 
            Table.AddColumn(AllData, "EventKey",
            (x) => 
                let
                    Val = if x[PrevEventNo] = x[EventNo] then null else AddToCounter()
                in 
                    Counter )
            in PrevRowComparison)

How would I go about creating and implementing a function that will increase the Counter value and then I can retrieve the counter value to place into the row?

I would like the output to be something like the image show below.

Index	EventNo	PrevEventNo	EventKey
0	1	1	0
1	1	1	0
2	3	1	1
3	3	3	1
4	3	3	1
5	4	3	2
6	4	4	2
7	1	4	3
8	3	1	4

[gabereal]

If anyone is interested, using @wdx223_Daniels solution. I did the following since I have nested tables.

    #"Table With EventKey" = Table.AddColumn(#"OriginalData", "EventKeyTable", each let
        AllData = [Custom],
        NewStep=Table.FromRecords(List.Accumulate(Table.ToRecords(AllData),{{},0},(x,y)=>let 
                 a=x{1}+Byte.From(y[EventNo]<>y[PrevEventNo]) in {x{0}&{y&[EventKey=a]},a}){0})
            in NewStep)

[wdx223_Daniel · Accepted solution]

NewStep=Table.FromRecords(List.Accmulate(Table.ToRecords(PreviousStepName),{{},0},(x,y)=>let a=x{1}+Byte.From(y[EventNo]<>y[PrevEventNo]) in {x{0}&{y&[EventKey=a]},a}){0})