multimap: nailed down type
add missing data
Switched to sets for indices
multimap: added implementations for multimap slice etc
small fix: handle all cases
separated out multimap and bag
extended typechecker with multimap
Added message to TBad and similar types
more work
more work
rename multimap
renamed imultimap
removed imap: it violates circularity
Compiles
fixed interpreter peek semantics to match old k3
fixed comparison error
flatten: fix implementation on empty collection
fixed another stupid comparison bug
switched over to map-of-list for frame env
attempt at bag functor