fn llsub(r: []Limb, a: []const Limb, b: []const Limb) void
fn llsub(r: []Limb, a: []const Limb, b: []const Limb) void { assert(a.len > b.len or (a.len == b.len and a[a.len - 1] >= b[b.len - 1])); assert(llsubcarry(r, a, b) == 0); }