I haven't found my way into cons-counting.... I know it's probably sloppy, but I allocate as though it takes constant time. Given that assumption, I'm less concerned with an overall constant-time slowdown and more concerned with allowing a '+ of N sequences of length L to take O(NL) time rather than O(N^2 L) time.
This is also a mostly[1] less limited design from an interface point of view. It's straightforward to port an extension from my other design to this one: The '+streamer and 'inside functions will be extended with (fn (x) x), and 'fn-add-to will be extended with the desired two-argument behavior. (I'd personally make a macro for this, and I'd use it to (re)implement the number-and-number case.)
[1] The exception is if you want to use '+ on streamer types themselves, since a '+streamer behavior of (fn (x) x) will cause that type to be confused with whatever other type wraps itself up as that streamer type.
