def _first_true_index(bools):
    """
    Given a pandas Series of bools, returns the index of the first occurrence
    of `True`. **Index-based, NOT location-based**
    I.e., say x = pd.Series({0: False, 2: False, 4: True}), then
    _first_true_index(x) will return index 4, not the positional index 2.

    If no value in `bools` is True, returns -1.
    """
    if not bools.any():
        return -1
    i_true = bools.idxmax()
    return i_true