in src/builder.rs [544:553]
fn rand<const W: usize>(s_dist: &impl Distribution<usize>) -> Equation<W> {
let mut rng = rand::thread_rng();
let s = s_dist.sample(&mut rng);
let mut a = [0u64; W];
for a_i in a.iter_mut() {
*a_i = rng.gen();
}
a[0] |= 1;
Equation::inhomogeneous(s, a, rng.gen::<u8>() & 1)
}