[R6RS] mutability of pairs

[William D Clinger]

Kent asked:
> Should we also say we're considering requiring that cons's
> second argument be a list?

It's okay with me to say we're considering it, but it's
orthogonal to mutability.  There's something to be said
for having an 8-byte representation that associates x
with y.  There is also something to be said for having
list operations that know their argument is a list.

Will