----------------------------------------------------------------------------- Disjunction NFA fragment: `a|b` 
Creates a disjunction choice between two fragments.
function orPair(first, second) {
  const inState = new NFAState();
  const outState = new NFAState();

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

  outState.accepting = true;
  first.out.accepting = false;
  second.out.accepting = false;

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

  return new NFA(inState, outState);
}