Luna, Forest's Essence

Luna, Forest's Essence enters the battlefield with
a +1/+1 counter on it.
Whenever another Unicorn you control leaves
the battlefield, if it had counters on it, you may
put those counters on Luna, Forest's Essence.
Whenever another Unicorn enters the battlefield
under your control, put a number of +1/+1
counters on it equal to the number of counters
on Luna, Forest's Essence