Thank you, Zaxo, for that reference.

Following up, I have found:

Augustus De Morgan originally observed that in classical propositional logic the following relationships hold:

not (P and Q) = (not P) or (not Q)

not (P or Q) = (not P) and (not Q)

This explains why the following are equivalent instructions:

print "no editor found\n"
   if ! (defined $ENV{EDITOR} || defined $ENV{VISUAL});

print "no editor found\n"
   if (!defined $ENV{EDITOR} && !defined $ENV{VISUAL});
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Thank you for setting me straight! This has been very helpful!