Function out_of_mask_fraction 

pub fn out_of_mask_fraction(
    samples: &[f64],
    rate: SampleRate,
    stft: StftParams,
    notes: &NoteList,
    params: MaskParams,
) -> Option<Normalized>

Returns the share of the signal’s power that lies outside every mask.

That share is the bleed of other instruments plus any note the list missed. Power is |X|^2 over all STFT cells; a frame where no note sounds counts as outside. Returns None for digital silence.