See DeMorgan's laws in formal logic. The solution is correct because (as truth tables will reveal), 

! (defined $ENV{EDITOR} || defined $ENV{VISUAL})

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is equivalent to, 
(! defined $ENV{EDITOR} && ! defined $ENV{VISUAL})

[download]

After Compline,
Zaxo

In reply to Re^2: interpretation of "neither...nor" by Zaxo
in thread Re: interpretation of "neither...nor" by cgmd

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