"col2_a": [0, 1, 0], "col2_b": [1, 0, 0],
    ...                    "col2_c": [0, 0, 1]})

    >>> df
          col1_a  col1_b  col2_a  col2_b  col2_c
    0       1       0       0       1       0
    1       0       1       1       0       0
    2       1       0       0       0       1

    >>> pd.from_dummies(df, sep="_")
        col1    col2
    0    a       b
    1    b       a
    2    a       c

    >>> df = pd.DataFrame({"col1_a": [1, 0, 0], "col1_b": [0, 1, 0],
    ...                    "col2_a": [0, 1, 0], "col2_b": [1, 0, 0],
    ...                    "col2_c": [0, 0, 0]})

    >>> df
          col1_a  col1_b  col2_a  col2_b  col2_c
    0       1       0       0       1       0
    1       0       1       1       0       0
    2       0       0       0       0       0

    >>> pd.from_dummies(df, sep="_", default_category={"col1": "d", "col2": "e"})
        col1    col2
    0    a       b
    1    b       a
    2    d       e
    r