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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 coefficient_between_units(unit_a, unit_b):
 """A helper to get the coefficient between two units. :param unit_a: The first unit. :type unit_a: safe.definitions.units :param unit_b: The second unit. :type unit_b: safe.definitions.units :return: The coefficient between these two units. :rtype: float """

    for mapping in unit_mapping:
        if unit_a == mapping[0] and unit_b == mapping[1]:
            return mapping[2]
        if unit_a == mapping[1] and unit_b == mapping[0]:
            return 1 / mapping[2]

    return None