Recently, I read that FreeBSD's /bin/sh fails:
http://www.freebsd.org/cgi/query-pr.cgi?pr=standards/45738
the IEEE Std 1003.1-2001:
http://www.opengroup.org/onlinepubs/007904975/utilities/unset.html
when it comes to the builtin unset.  tcsh and bash do follow it.

I then checked zsh.  It also fails this specification (if it desired to
follow it for unset?).  I have attached a patch that returns zero for
unset.  It also affects unhash for functions to handle unset -f.

Sean

P.S. I am not on the list; please cc.

P.P.S.  http://www.zsh.org/mla/patches.shtml has not been updated past
March 2004.
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sean@farley.org