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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 gunzip(gzip_file, file_gunzip=None):
 """Unzip .gz file. Return filename of unzipped file."""

    if file_gunzip is None:
        file_gunzip = os.path.splitext(gzip_file)[0]
        gzip_open_to(gzip_file, file_gunzip)
        return file_gunzip