Adds one or more key-value pairs to an association variable in-place
AssociateTo[a, key -> val]
AssociateTo[a, {key1 -> val1, key2 -> val2, …}]
assoc = Association[{a -> 1, b -> 2, c -> 3}]; AssociateTo[assoc, a -> 11]; AssociateTo[assoc, d -> 4]
→ <|a -> 11, b -> 2, c -> 3, d -> 4|>