Who said 'Be who you is - if you is who you ain't you ain't who you is'?

The quote has been attributed to painter (and career coach) Ian Summers, but may not have originated with him.
According to writer/filmaker Phil Cousineau, it is the epitaph on a gravemarker an Boothill Cemetery in Tombstone, AZ.

"Be who you is, cuz if you be what you ain't, then you ain't what you is."

Source: The Hero's Journey: Joseph Campbell On His Life And Work, Harper & Row, 1990.
I think the quote is "Be what you is, 'cos if you be what you ain't, you ain't what you is". the quite is referenced in the (I think) following book by Joseph Campbell "The Heros' Journey: Joseph Campbell on his life and works", where he attributes it to an epitath on an unmarked grave in a Kansas (or some western state) town's graveyard. it is a brilliant quote - as soon as I read it, I thought - he's got it in one! What more need you say! I marked the page in the book, so I could go back and check it, but I think it's all the better for being anonymous. Peter