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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 _set_default_key(mapping):
 """Replace the field with the most features with a 'default' field."""

        key_longest = sorted([(len(v), k) for k, v in mapping.items()],
                             reverse=True)[0][1]
        mapping['default'] = key_longest
        del (mapping[key_longest])