Function bit_serial_sub 

pub fn bit_serial_sub(
    df: &mut DramDataflow,
    a: &[usize],
    b: &[usize],
    ones_carry: usize,
) -> Vec<usize>

Bit-serial two’s-complement SUBTRACT: a - b = a + NOT(b) + 1 (mod 2^B). Negates each b plane and ripple-adds with carry-in 1 (ones_carry = a row with every lane’s bit set). Returns the B difference-plane rows. Arithmetic subtraction, still composed from the boolean basis in-DRAM.