[plt-scheme] Is there a set data-type?

I feel like I started a tempest in a teapot.

I really just needed to check that an operation passed into a function
(as a symbol) was one of the permitted operations. Roughly 10 members,
but I didn't want to read through the list of all the operations each
time I had to check. Thus, the question about (in my mind, finite)
sets and my presumption that a hashtable would be a good
representation.

Sorry for not asking a more specific question and encouraging all this
discussion about much more esoteric topics than I had intended to. :-)

Todd