--
disc :: Position -> R -> sh :~> Position
disc (originX, originY, originZ) radiusMax _ix gen
  = do  radius          <- uniformR (0,radiusMax) gen
        theta           <- uniformR (0, pi)       gen
        phi             <- uniformR (0, 2*pi)     gen

        return ( originX + radius * sin theta * cos phi
               , originY + radius * sin theta * sin phi
               , originZ + radius * cos theta )