ln -f

[Guy Harris]

>Why don't the versions of ln that you know of on 4.3 BSD and SunOS 4
>use the rename system call?

Because they're named "ln", not "mv".

"rename()" is *NOT* an atomic operation that removes the target and
makes an *additional* link from the source with the name of the target. 
It's an atomic operation that *renames* the source to the target, and if
it succeeds does *not* leave the source behind.