"Can't happen"

• One kind of "can't happen" looks like this:

           if ( $mnt eq 'Jan'){ $mt = 1 }
           elsif ( $mnt eq 'Feb'){$mt = 2 } 
       ...
           elsif ( $mnt eq 'Dec'){$mt = 12 }

• Where's the else clause?

• "Oh, that can't happen"

  • ... unless the input data is malformed