Here are a few things that you need to know:
1. Define "struct dag_list_node_t HEAD = {NULL, NULL}" in all the applications.
   This is used in libdag as the HEAD node for the DAG that it constructs from
   the DOT file. 

2. Define "int num_vertices". This is used to hold the number of vertices in 
   libdag.

3. Before calling yyparse(), please ensure that you have opened the desired 
   DOT file as "stdin".
