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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 mask_coverage(coverage: mx.sym.Symbol, source_length: mx.sym.Symbol) -> mx.sym.Symbol: """ Masks all coverage scores that are outside the actual sequence. :param coverage: Input coverage vector. Shape: (batch_size, seq_len, coverage_num_hidden). :param source_length: Source length. Shape: (batch_size,). :return: Masked coverage vector. Shape: (batch_size, seq_len, coverage_num_hidden). """

    return mx.sym.SequenceMask(data=coverage, axis=1, use_sequence_length=True, sequence_length=source_length)