Looks up a key in an association, returning a default value if the key is
Lookup[assoc, key]
Lookup[assoc, {key1, key2, …}]
Lookup[{assoc1, assoc2, …}, key]
Lookup[assoc, key, default]
Lookup[assoc, keys, default, h]
Lookup[key]
Lookup[<|a -> 1, b -> 2|>, a]
→ 1Lookup[<|a -> 1, b -> 2|>, c, "missing"]
→ missingLookup[a][<|a -> 1, b -> 2|>]
→ 1