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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 is_symbol(string):
 """ Return true if the string is a mathematical symbol. """

    return (
        is_int(string) or is_float(string) or
        is_constant(string) or is_unary(string) or
        is_binary(string) or
        (string == '(') or (string == ')')
    )