Unity Coroutines

using UnityEngine;

public class CoroutineExample : MonoBehaviour
{
    /*
    IEnumerator Start()
    {   //start can be used as an IEnumerator, unlike awake
    }
    */
    int a = 0;
    void Start()
    {
        Foo(2); //won't execute because you need to call it with StartCoroutine unless you're already in an IEnumerator

        Print("beginning of start(): should be 0");
        StartCoroutine(Foo(2));
        Print("end of start(): should be 0");
    }
    IEnumerator Foo(int seconds)
    {
        Print("beginning of Foo(): should be 0");
        yield return new WaitForSeconds(seconds); //waitforseconds is builtin unity command
        Print("we wait for the above command: should be "+seconds);

        yield return Bar(seconds * 2);
    }
    IEnumerator Bar(int seconds)
    {
        string note;

        note = "beginning of Bar(): this happens after the WaitForSeconds in Foo";
        Print(note);

        note = "starting IEnumerator inside IEnumerator with yield return: ok, and it waits";
        yield return PrintNoteAndWait1Second(note);

        note = "starting IEnumerator inside IEnumerator: ok, but doesn't wait";
        PrintNoteAndWait1Second(note);

        note = "starting IEnumerator inside IEnumerator with StartCoroutine: ok, but doesn't wait";
        StartCoroutine(PrintNoteAndWait1Second(note));


        Coroutine coroutine = PrintNoteAndWait1Second("you can hold a reference to a couroutine");
    }
    IEnumerator PrintNoteAndWait1Second(string note)
    {
        Print(note);
        yield return new WaitForSeconds(1);
    }

    void Print(string note)
    {
        Debug.Log(String.Format("a: {0} @ {1}\n\t>{2}", a, Time.time, note));
        a++;
    }
}