Prepend a level to a pandas MultiIndex

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I have a DataFrame with a MultiIndex created after some grouping:

import numpy as np
import pandas as p
from numpy.random import randn

df = p.DataFrame({
    "A" : ["a1", "a1", "a2", "a3"]
  , "B" : ["b1", "b2", "b3", "b4"]
  , "Vals" : randn(4)
}).groupby(["A", "B"]).sum()

df

Output>            Vals
Output> A  B           
Output> a1 b1 -1.632460
Output>    b2  0.596027
Output> a2 b3 -0.619130
Output> a3 b4 -0.002009

How do I prepend a level to the MultiIndex so that I turn it into something like:

Output>                       Vals
Output> FirstLevel A  B           
Output> Foo        a1 b1 -1.632460
Output>               b2  0.596027
Output>            a2 b3 -0.619130
Output>            a3 b4 -0.002009

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