class biquad_filter : int -> [ `All_pass | `Band_pass | `High_pass | `High_shelf | `Low_pass | `Low_shelf | `Notch | `Peaking ] -> ?gain:float -> float -> float -> t
int -> [ `All_pass | `Band_pass | `High_pass | `High_shelf | `Low_pass | `Low_shelf | `Notch | `Peaking ] -> ?gain:float -> float -> float ->
t