[[ ! x || x ]]
echo returns: $?
+# parenthesized terms didn't work right until post-2.04
+[[ a ]]
+echo returns: $?
+
+[[ (a) ]]
+echo returns: $?
+
+[[ -n a ]]
+echo returns: $?
+
+[[ (-n a) ]]
+echo returns: $?
+
# unset variables don't need to be quoted
[[ -n $UNSET ]]
echo returns: $?