[plt-dev] `unsafe-fl' and unboxing

At Sat, 3 Oct 2009 08:34:03 -0600, Matthew Flatt wrote:
> (while the contract on the "checked" version ensures that the unsafe
> operations will not cause a crash).

Not true. Sam points out that a vector (or other sequence) can be
mutable, so checking elements at the beginning does not make `variance'
safe if it uses unsafe operations on the elements internally.