Set#&

static VALUE
set_i_intersection(VALUE set, VALUE other)
{
    VALUE new_set = set_s_alloc(rb_obj_class(set));
    set_table *stable = RSET_TABLE(set);
    set_table *ntable = RSET_TABLE(new_set);

    if (rb_obj_is_kind_of(other, rb_cSet)) {
        set_table *otable = RSET_TABLE(other);
        if (set_table_size(stable) >= set_table_size(otable)) {
            /* Swap so we iterate over the smaller set */
            otable = stable;
            set = other;
        }

        struct set_intersection_data data = {
            .set = new_set,
            .into = ntable,
            .other = otable
        };
        set_iter(set, set_intersection_i, (st_data_t)&data);
    }
    else {
        struct set_intersection_data data = {
            .set = new_set,
            .into = ntable,
            .other = stable
        };
        rb_block_call(other, enum_method_id(other), 0, 0, set_intersection_block, (VALUE)&data);
    }

    return new_set;
}