code: public static <S, A> State<S, A> state(A a) {
          return gets(constantly(a));
      }
docs: Create a {@link State} that returns <code>a</code> as its result and its initial state as its final state.
  
  @param a   the result
  @param <S> the state type
  @param <A> the result type
  @return the new {@link State} instance