# File lib/png.rb, line 441
    def []= x, y, color
      raise "bad x value #{x} >= #{@width}" if x >= @width
      raise "bad y value #{y} >= #{@height}"  if y >= @height
      raise "bad color #{color.inspect}" unless color.kind_of? PNG::Color
      @data[@height-y-1][x] = color
    end