In a previous exercise, you collected instances of prize categories not being awarded in particular years. You implemented this using a for loop in Python. You will now implement this as an aggregation pipeline that:

Filters for original prize categories (i.e. sans economics),
Projects category and year,
Groups distinct prize categories awarded by year,
Projects prize categories not awarded by year,
Filters for years with missing prize categories, and
Returns a cursor of documents in reverse chronological order, one per year, each with a list of missing prize categories for that year.

Remember to use field paths (precede field names with "$") to extract field values in expressions.

Something Extra: $addFields to Aid Analysis

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