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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 previous_key(tuple_of_tuples, key):
 """Returns the key which comes before the give key. It Processes a tuple of 2-element tuples and returns the key which comes before the given key. """

    for i, t in enumerate(tuple_of_tuples):
        if t[0] == key:
            try:
                return tuple_of_tuples[i - 1][0]
            except IndexError:
                return None