For example, I have a nested association like this
<|"fff" -> <|"2001" -> <|5040.` -> {"S20010037", "S20010038",
"S20010039", "S20010040", "S20010041", "S20010042"}|>,
"2005" -> <|4350.` -> {"S20050448", "S20050449"},
3450.` -> {"S20050998", "S20050999"}|>|>|>
I want to "Flatten" it like this
<|{fff, 2001, 5040.} -> {"S20010037", "S20010038", "S20010039",
"S20010040", "S20010041", "S20010042"}, {fff, 2005,
4350.} -> {"S20050448", "S20050449"}, {fff, 2005,
3450.} -> {"S20050998", "S20050999"}|>
I can't figure out a good way. How to do it elegantly?