But if it has to be able to be mutated in some cases, you're stuck. This is a place where Scheme's parameterize may be useful: Define a closure with the setter in scope, and write it so that when it's called with a lambda as an arg, it will raise an error unless those certain conditions are met, in which case, it will use parameterize to make the setter available in the lambda's scope. Or I suppose it could pass it in, which would be slightly simpler...
