code: def prev(self, event):
      self.ind -= 1
      i = self.ind % len(freqs)
      ydata = np.sin(2 * np.pi * freqs[i] * t)
      l.set_ydata(ydata)
      plt.draw()
  
imports: import matplotlib.pyplot
  import numpy
  from matplotlib.widgets import Button
label_desc: Use image node_id 2 for calling the Index obj's underlying member method code with example usage: obj.prev(event) without return types