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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 keys_in_dict(d, parent_key, keys):
 """ Create a list of keys from a dict recursively. """

    for key, value in d.iteritems():
        if isinstance(value, dict):
            keys_in_dict(value, key, keys)
        else:
            if parent_key:
                prefix = parent_key + "."
            else:
                prefix = ""

            keys.append(prefix + key)

    return keys