----------------------------------------------------------------------------- Alteration NFA fragment: `abc` 
Creates a connection between two NFA fragments on epsilon transition.

[in-a] --a--> [out-a] --ε--> [in-b] --b--> [out-b]
function altPair(first, second) {
  first.out.accepting = false;
  second.out.accepting = true;

  first.out.addTransition(EPSILON, second.in);

  return new NFA(first.in, second.out);
}