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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 chi_squared(source_frequency, target_frequency):
 """Calculate the Chi Squared statistic by comparing ``source_frequency`` with ``target_frequency``. Example: 0.1 Args: source_frequency (dict):
 Frequency map of the text you are analyzing target_frequency (dict):
 Frequency map of the target language to compare with Returns: Decimal value of the chi-squared statistic """

    # Ignore any symbols from source that are not in target.
    # TODO: raise Error if source_len is 0?
    target_prob = frequency_to_probability(target_frequency)
    source_len = sum(v for k, v in source_frequency.items() if k in target_frequency)

    result = 0
    for symbol, prob in target_prob.items():
        symbol_frequency = source_frequency.get(symbol, 0)  # Frequecy is 0 if it doesnt appear in source
        result += _calculate_chi_squared(symbol_frequency, prob, source_len)

    return result