Function pio_reference_bus 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn pio_reference_bus(
    net: *const PioNetwork,
) -> isize

Dense [0, n) index of the single reference bus, or -1 if not exactly one (also -1 if the index is too large for isize). A network may carry several references (one per island, or a normalized case that kept the file’s multiple REF buses); use pio_n_reference_buses to tell zero from many, and pio_reference_buses to read them all.