Unravel cumulative totals to their initial elements in Power BI and Power Query

Recently I came across an interesting request where someone wanted to un-cumulate their quarterly YTD-figures (green) into their single quarters values (red) like so (“Unravel cumulative totals”):

Task

Retrieve every Quarters Amount from the Quarter To Date values (“YAmount”)

Method

To retrieve this value, one would have to start with the first value in the year. This is also the value of the first quarter, but for the 2nd quarter, one would have to deduct the value of the first quarter from the cumulative value of the 2nd quarter. So basically retrieving the previous cumulative row and deduct it from the current cumulative row. Do this for every row, unless it’s the start of the year or belongs to a different account code in this example:

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