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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def screenshot(filename="screenshot.png"):
 """ Save a screenshot of the current rendering window. """

    if not settings.plotter_instance.window:
        colors.printc('~bomb screenshot(): Rendering window is not present, skip.', c=1)
        return
    w2if = vtk.vtkWindowToImageFilter()
    w2if.ShouldRerenderOff()
    w2if.SetInput(settings.plotter_instance.window)
    w2if.ReadFrontBufferOff()  # read from the back buffer
    w2if.Update()
    pngwriter = vtk.vtkPNGWriter()
    pngwriter.SetFileName(filename)
    pngwriter.SetInputConnection(w2if.GetOutputPort())
    pngwriter.Write()