Why does Lung the Quick's tombstone read "No Unicorns in Heaven"?

Question

I'm reading *Digger*, and there's a scene in chapter six where Mural is crying next to a tombstone that reads "No Unicorns in Heaven."What does the tombstone mean?

Answer 1

In The Last Unicorn by Peter S. Beagle he states early on that unicorns are immortal. Does that help?