Function would_modify_words 

fn would_modify_words<Op>(lhs: &[u64], rhs: &[u64], op: Op) -> bool
where
    Op: Fn(u64, u64) -> u64,

Returns true if a call to update_words would modify lhs, i.e. lhs[i] != op(lhs[i], rhs[i]) for some i.