perl -e"print('Do I understand? (yes/no): ');my $understand=<>;my $do_
+not; $do_not='do not ' if ($understand!~/^yes/i); print($/,'If you ',
+$do_not,'understand, things are such as they are.',$/)"
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Do I understand? (yes/no): may be

If you do not understand, things are such as they are.