*
         * TODO(cpovirk): Consider checking onlyOnRight.containsKey instead of right.containsKey.
         * That could change behavior if the input maps use different equivalence relations (and so
         * 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.

         *
         * TODO(cpovirk): Consider checking onlyOnRight.containsKey instead of right.containsKey.
         * That could change behavior if the input maps use different equivalence relations (and so
         * 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.