how do I add 'd' to the index below without having to reset it first?

from pandas import DataFrame

df = DataFrame({'a': range(6), 'b': range(6), 'c': range(6)})
df.set_index(['a','b'], inplace=True)
df['d'] = range(6)

# how do I set index to 'a b d' without having to reset it first?
df.reset_index(['a','b','d'], inplace=True)
df.set_index(['a','b','d'], inplace=True)

df
python dataframe pandas
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