immutable types

[Skaperen]

in that language i created 3 decades ago, its mapping type also served as sets. it had a way to place a key with no value (and to delete a value while keeping the key in place). every type had mutable and immutable forms. i remember when i first encountered Python, i backed away because str was strictly immutable. now i realize that every use case for a "mutable str" is better handled by a list.