I'll disagree here an say a form like... 
if( !$x && !$y &&  $z or
    !$x &&  $y && !$z or
     $x && !$y && !$z    )
{
    print "Exactly one...";
}

[download]
...is easier to maintain because it is more explicit as to what is required for a truth value. Anyone versed in boolean algebra will easily recognize the sum-of-product form here and instantly grasp the intent.

In reply to Re^3: One out of three ain't bad by Anonymous Monk
in thread One out of three ain't bad by saintmike

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