code: def check_coords(x, y)
  			if 0 <= x && x < 8 && 0 <= y && y < 8
  				true
  			else
  				fail ArgumentError, "coord (#{x},#{y}) outside of permitted range ((0..7), (0..7))"
  			end
  		end
docs: Verify supplied coords +x+ and +y+
  
   Raises ArgumentError if the supplied coords are invalid
   (doesn't address configured pixel)