After you implement your code in nfa.ml, you can run the public tests as:

   ocaml public_RE_to_str.ml
   ocaml public_str_to_RE.ml
   ocaml public_NFA_step.ml    
   ocaml public_NFA_accept.ml  
   ocaml public_RE_to_NFA.ml  
   ocaml public_str_to_NFA.ml  

The outputs of each code should match the following files, respectively: 

   outputs/public_RE_to_str.out
   outputs/public_str_to_RE.out
   outputs/public_NFA_step.out
   outputs/public_NFA_accept.out
   outputs/public_RE_to_NFA.out
   outputs/public_str_to_NFA.out

Some code is provided in testUtils.ml to help print out data structures.
You do not/should not use code in testUtils.ml in your code, it is there
solely for the purpose of testing.