code: def testExtractNonZeroStart(self):
      testing_lib.add_track_to_sequence(
          self.note_sequence, 0, [(60, 100, 0.0, 4.0)]
      )
      quantized_sequence = sequences_lib.quantize_note_sequence(
          self.note_sequence, steps_per_quarter=1
      )
  
      seqs, _ = polyphony_lib.extract_polyphonic_sequences(
          quantized_sequence, start_step=4, min_steps_discard=1
      )
      self.assertEqual(0, len(seqs))
      seqs, _ = polyphony_lib.extract_polyphonic_sequences(
          quantized_sequence, start_step=0, min_steps_discard=1
      )
      self.assertEqual(1, len(seqs))
  
imports: import copy
  from magenta.common import testing_lib
  from magenta.models.polyphony_rnn import polyphony_lib
  from note_seq import sequences_lib
  from note_seq import testing_lib
  from note_seq.protobuf import music_pb2
  import tensorflow.compat.v1
label_desc: Use image node_id 22 for calling the PolyphonyLibTest obj's underlying member method code with example usage: obj.testExtractNonZeroStart() without return types