/ranreplace command

Have a command that randomly replaces a certain block with another block. Would be useful for texturing and for map makers creating mining style games.

/Ranreplace [Coordinates] [Coordinates] [Block to replace] [Block to replace with] [rarity]

Rarity being 1 in however many blocks. If I wanted 1 in 10 I would enter 10. If I wanted 1 in 100 I would enter 100.