pop_rl
struct range_list *pop_rl(struct range_list_stack **rl_stack);
right = pop_rl(&rl_stack);
left = pop_rl(&rl_stack);
pop_rl(&rl_stack);
*rl = pop_rl(&rl_stack);
rl = pop_rl(rl_stack);
pop_rl(&remaining_cases);