[plt-scheme] PLT v 4.0 discussion - essential data structures

PLT v 4.0 has a much stronger sense of "batteries included", but there
are still many data structures that are standard in other languages,
and lacking in PLT Scheme.  Some of these are addressed via planet
packages or srfis, but many of these aren't tightly integrated with
other aspects of PLT Scheme, such as match and comprehensions.

I'm curious -- what are the data structures that others feel would be
worthy of first-class status?

For me, I'd say the biggest things I'm missing are streams, an
implementation of both mutable and immutable sets analagous to
hash-tables, extensible vectors, and mutable/immutable deque options.

--Mark