human
Solve the following python programming problem given the following code:
def produce_parser_topo_sorting(hlir) : 
    header_graph = Graph () 
    def walk_rec(hlir, parse_state, prev_hdr_node, tag_stacks_index) : 
        assert isinstance (parse_state, p4_parse_state) 
        for call in parse_state.call_sequence : 
            call_type = call [0] 
            if (call_type == parse_call.extract) : 
                hdr = call [1] 
                if hdr.virtual : 
                    base_name = hdr.base_name 
                    current_index = tag_stacks_index [base_name] 
                    if (current_index > hdr.max_index) : 
                        return 
tag_stacks_index [base_name] += 1 
                    name = (base_name + ('[%d]' % current_index)) 
                    hdr = hlir.p4_header_instances [name] 
if (hdr not in header_graph) : 
                    header_graph.add_node (hdr) 
hdr_node = header_graph.get_node (hdr) 
                if prev_hdr_node : 
                    prev_hdr_node.add_edge_to (hdr_node) 
else : 
                    header_graph.root = hdr 
prev_hdr_node = hdr_node 
for (branch_case, next_state) in parse_state.branch_to.items () : 
            if (not next_state) : 
                continue 
if (not isinstance (next_state, p4_parse_state)) : 
                continue 
walk_rec (hlir, next_state, prev_hdr_node, tag_stacks_index.copy ()) 
start_state = hlir.p4_parse_states ['start'] 
    walk_rec (hlir, start_state, None, defaultdict (int)) 
    header_topo_sorting = header_graph.produce_topo_sorting () 
    return hlir 

    
    

    Fix the buggy line: return hlir
assistant
#Here is Python program to fix the buggy line: return hlir
.

return header_topo_sorting