Mutates an item in a collection.

(access= c k nv) is usually written as (= [c k] nv), using the = macro.

Possible values for coll and key, and support for the ? syntax and slicing syntax, are all identical to the access function.

When assigning to a slice, new-val must be a deque. It does not need to have the same length as the slice which is being replaced.

(= [dst 1 : 3] src)

; ...is roughly equivalent to...

(del! dst 1 : 3)
(insert! dst 1 ..src)

It's an error to mutate a collection which has previously been frozen.