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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 isValid(folder, epoch=0):
 """ Check if the given folder is a valid preprocessed dataset """

    # Validate by checking for the training 'silence.pkl' on the given epoch
    # This file is unique to our pre-processed dataset generated by 'process_dataset.py'
    return os.path.exists(os.path.join(folder, str(epoch), "train", "silence.pkl"))