when you have to turn into a chrysalis--you will some day, you
know--and then after that into a butterfly, I should think you'll
feel it a little queer, won't you?'

  `Not a bit,' said the Caterpillar.

  `Well, perhaps your feelings may be different,' said Alice;
`all I know is, it would feel very queer to ME.'

  `You!' said the Caterpillar contemptuously.  `Who are YOU?'

when you have to turn into a chrysalis--you will some day, you
know--and then after that into a butterfly, I should think you'll
feel it a little queer, won't you?'

  `Not a bit,' said the Caterpillar.

  `Well, perhaps your feelings may be different,' said Alice;
`all I know is, it would feel very queer to ME.'

  `You!' said the Caterpillar contemptuously.  `Who are YOU?'

         * a key that appears once in `right` might appear multiple times in `left`). We don't
         * guarantee behavior in that case, anyway, and the current behavior is likely undesirable.
         * So that's either a reason to feel free to change it or a reason to not bother thinking
         * further about this.
         */
        V rightValue = uncheckedCastNullableTToT(onlyOnRight.remove(leftKey));