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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 parse(string, language=None):
 """ Return a solution to the equation in the input string. """

    if language:
        string = replace_word_tokens(string, language)

    tokens = tokenize(string)
    postfix = to_postfix(tokens)

    return evaluate_postfix(postfix)