Function ground_rule 

pub fn ground_rule(
    template: &AspRule,
    var: u64,
    domain: &[u64],
    out: &mut [AspRule],
) -> usize

Ground a rule TEMPLATE by substituting variable var with each element of domain, writing the ground instances into out. Returns the count. Apply repeatedly (over the partially-ground output) for multiple variables. Zero-heap — bounded by out.len() (the “millions of constraints” ceiling is the caller’s buffer, not a heap allocation here).