Function findInterval 

pub unsafe extern "C" fn findInterval(
    xt: *mut f64,
    n: c_int,
    x: f64,
    rightmost_closed: Rboolean,
    all_inside: Rboolean,
    ilo: c_int,
    mflag: *mut c_int,
) -> c_int

../../appl/interv.c: also in Applic.h