Immutable things.

[Chris Bitmead uid(x22068)]

I think all this talk of deciding which functions do and don't return
immutable things is foolish. What are you going to do? Have a
map-car-immutable which returns an immutable list and map-car-mutable
which returns a mutable one? Are you going to double up on every
function in Scheme or CL?

Sure you could just have the standard map-car return an immutable list
and force functional programming, but that is really taking
incompatibility to an extreme. Are you sure you want to enforce this?
Perhaps you should be building a Haskell OS instead of a LispOS.

Besides which I know that immutable/mutable objects places another
level of burden on the programmer. It can be quite a hassle in C++.

If you must have immutable things I encourage some function which
makes an existing object immutable, possibly recursively through
structures.