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Lookup

Looks up a key in an association, returning a default value if the key is absent.

$ wo 'Lookup[<|a -> 1, b -> 2|>, a]'
1

$ wo 'Lookup[<|a -> 1, b -> 2|>, c, "missing"]'
missing

The operator form Lookup[key] applies to an association later.

$ wo 'Lookup[a][<|a -> 1, b -> 2|>]'
1