diff --git a/concurrency/feedreader/main.go b/concurrency/feedreader/main.go
new file mode 100644
index 0000000..6be5499
--- /dev/null
+++ b/concurrency/feedreader/main.go
@@ -0,0 +1,610 @@
+// +build OMIT
+
+// realmain runs the Subscribe example with a real RSS fetcher.
+package main
+
+import (
+	"fmt"
+	"math/rand"
+	"time"
+
+	rss "github.com/jteeuwen/go-pkg-rss"
+)
+
+// STARTITEM OMIT
+// An Item is a stripped-down RSS item.
+type Item struct{ Title, Channel, GUID string }
+
+// STOPITEM OMIT
+
+// STARTFETCHER OMIT
+// A Fetcher fetches Items and returns the time when the next fetch should be
+// attempted.  On failure, Fetch returns a non-nil error.
+type Fetcher interface {
+	Fetch() (items []Item, next time.Time, err error)
+}
+
+// STOPFETCHER OMIT
+
+// STARTSUBSCRIPTION OMIT
+// A Subscription delivers Items over a channel.  Close cancels the
+// subscription, closes the Updates channel, and returns the last fetch error,
+// if any.
+type Subscription interface {
+	Updates() <-chan Item
+	Close() error
+}
+
+// STOPSUBSCRIPTION OMIT
+
+// STARTSUBSCRIBE OMIT
+// Subscribe returns a new Subscription that uses fetcher to fetch Items.
+func Subscribe(fetcher Fetcher) Subscription {
+	s := &sub{
+		fetcher: fetcher,
+		updates: make(chan Item),       // for Updates
+		closing: make(chan chan error), // for Close
+	}
+	go s.loop()
+	return s
+}
+
+// STOPSUBSCRIBE OMIT
+
+// sub implements the Subscription interface.
+type sub struct {
+	fetcher Fetcher         // fetches items
+	updates chan Item       // sends items to the user
+	closing chan chan error // for Close
+}
+
+// STARTUPDATES OMIT
+func (s *sub) Updates() <-chan Item {
+	return s.updates
+}
+
+// STOPUPDATES OMIT
+
+// STARTCLOSE OMIT
+// STARTCLOSESIG OMIT
+func (s *sub) Close() error {
+	// STOPCLOSESIG OMIT
+	errc := make(chan error)
+	s.closing <- errc // HLchan
+	return <-errc     // HLchan
+}
+
+// STOPCLOSE OMIT
+
+// loopCloseOnly is a version of loop that includes only the logic
+// that handles Close.
+func (s *sub) loopCloseOnly() {
+	// STARTCLOSEONLY OMIT
+	var err error // set when Fetch fails
+	for {
+		select {
+		case errc := <-s.closing: // HLchan
+			errc <- err      // HLchan
+			close(s.updates) // tells receiver we're done
+			return
+		}
+	}
+	// STOPCLOSEONLY OMIT
+}
+
+// loopFetchOnly is a version of loop that includes only the logic
+// that calls Fetch.
+func (s *sub) loopFetchOnly() {
+	// STARTFETCHONLY OMIT
+	var pending []Item // appended by fetch; consumed by send
+	var next time.Time // initially January 1, year 0
+	var err error
+	for {
+		var fetchDelay time.Duration // initally 0 (no delay)
+		if now := time.Now(); next.After(now) {
+			fetchDelay = next.Sub(now)
+		}
+		startFetch := time.After(fetchDelay)
+
+		select {
+		case <-startFetch:
+			var fetched []Item
+			fetched, next, err = s.fetcher.Fetch()
+			if err != nil {
+				next = time.Now().Add(10 * time.Second)
+				break
+			}
+			pending = append(pending, fetched...)
+		}
+	}
+	// STOPFETCHONLY OMIT
+}
+
+// loopSendOnly is a version of loop that includes only the logic for
+// sending items to s.updates.
+func (s *sub) loopSendOnly() {
+	// STARTSENDONLY OMIT
+	var pending []Item // appended by fetch; consumed by send
+	for {
+		var first Item
+		var updates chan Item // HLupdates
+		if len(pending) > 0 {
+			first = pending[0]
+			updates = s.updates // enable send case // HLupdates
+		}
+
+		select {
+		case updates <- first:
+			pending = pending[1:]
+		}
+	}
+	// STOPSENDONLY OMIT
+}
+
+// mergedLoop is a version of loop that combines loopCloseOnly,
+// loopFetchOnly, and loopSendOnly.
+func (s *sub) mergedLoop() {
+	// STARTFETCHVARS OMIT
+	var pending []Item
+	var next time.Time
+	var err error
+	// STOPFETCHVARS OMIT
+	for {
+		// STARTNOCAP OMIT
+		var fetchDelay time.Duration
+		if now := time.Now(); next.After(now) {
+			fetchDelay = next.Sub(now)
+		}
+		startFetch := time.After(fetchDelay)
+		// STOPNOCAP OMIT
+		var first Item
+		var updates chan Item
+		if len(pending) > 0 {
+			first = pending[0]
+			updates = s.updates // enable send case
+		}
+
+		// STARTSELECT OMIT
+		select {
+		case errc := <-s.closing: // HLcases
+			errc <- err
+			close(s.updates)
+			return
+			// STARTFETCHCASE OMIT
+		case <-startFetch: // HLcases
+			var fetched []Item
+			fetched, next, err = s.fetcher.Fetch() // HLfetch
+			if err != nil {
+				next = time.Now().Add(10 * time.Second)
+				break
+			}
+			pending = append(pending, fetched...) // HLfetch
+			// STOPFETCHCASE OMIT
+		case updates <- first: // HLcases
+			pending = pending[1:]
+		}
+		// STOPSELECT OMIT
+	}
+}
+
+// dedupeLoop extends mergedLoop with deduping of fetched items.
+func (s *sub) dedupeLoop() {
+	const maxPending = 10
+	// STARTSEEN OMIT
+	var pending []Item
+	var next time.Time
+	var err error
+	var seen = make(map[string]bool) // set of item.GUIDs // HLseen
+	// STOPSEEN OMIT
+	for {
+		// STARTCAP OMIT
+		var fetchDelay time.Duration
+		if now := time.Now(); next.After(now) {
+			fetchDelay = next.Sub(now)
+		}
+		var startFetch <-chan time.Time // HLcap
+		if len(pending) < maxPending {  // HLcap
+			startFetch = time.After(fetchDelay) // enable fetch case  // HLcap
+		} // HLcap
+		// STOPCAP OMIT
+		var first Item
+		var updates chan Item
+		if len(pending) > 0 {
+			first = pending[0]
+			updates = s.updates // enable send case
+		}
+		select {
+		case errc := <-s.closing:
+			errc <- err
+			close(s.updates)
+			return
+		// STARTDEDUPE OMIT
+		case <-startFetch:
+			var fetched []Item
+			fetched, next, err = s.fetcher.Fetch() // HLfetch
+			if err != nil {
+				next = time.Now().Add(10 * time.Second)
+				break
+			}
+			for _, item := range fetched {
+				if !seen[item.GUID] { // HLdupe
+					pending = append(pending, item) // HLdupe
+					seen[item.GUID] = true          // HLdupe
+				} // HLdupe
+			}
+			// STOPDEDUPE OMIT
+		case updates <- first:
+			pending = pending[1:]
+		}
+	}
+}
+
+// loop periodically fecthes Items, sends them on s.updates, and exits
+// when Close is called.  It extends dedupeLoop with logic to run
+// Fetch asynchronously.
+func (s *sub) loop() {
+	const maxPending = 10
+	type fetchResult struct {
+		fetched []Item
+		next    time.Time
+		err     error
+	}
+	// STARTFETCHDONE OMIT
+	var fetchDone chan fetchResult // if non-nil, Fetch is running // HL
+	// STOPFETCHDONE OMIT
+	var pending []Item
+	var next time.Time
+	var err error
+	var seen = make(map[string]bool)
+	for {
+		var fetchDelay time.Duration
+		if now := time.Now(); next.After(now) {
+			fetchDelay = next.Sub(now)
+		}
+		// STARTFETCHIF OMIT
+		var startFetch <-chan time.Time
+		if fetchDone == nil && len(pending) < maxPending { // HLfetch
+			startFetch = time.After(fetchDelay) // enable fetch case
+		}
+		// STOPFETCHIF OMIT
+		var first Item
+		var updates chan Item
+		if len(pending) > 0 {
+			first = pending[0]
+			updates = s.updates // enable send case
+		}
+		// STARTFETCHASYNC OMIT
+		select {
+		case <-startFetch: // HLfetch
+			fetchDone = make(chan fetchResult, 1) // HLfetch
+			go func() {
+				fetched, next, err := s.fetcher.Fetch()
+				fetchDone <- fetchResult{fetched, next, err}
+			}()
+		case result := <-fetchDone: // HLfetch
+			fetchDone = nil // HLfetch
+			// Use result.fetched, result.next, result.err
+			// STOPFETCHASYNC OMIT
+			fetched := result.fetched
+			next, err = result.next, result.err
+			if err != nil {
+				next = time.Now().Add(10 * time.Second)
+				break
+			}
+			for _, item := range fetched {
+				if id := item.GUID; !seen[id] { // HLdupe
+					pending = append(pending, item)
+					seen[id] = true // HLdupe
+				}
+			}
+		case errc := <-s.closing:
+			errc <- err
+			close(s.updates)
+			return
+		case updates <- first:
+			pending = pending[1:]
+		}
+	}
+}
+
+// naiveMerge is a version of Merge that doesn't quite work right.  In
+// particular, the goroutines it starts may block forever on m.updates
+// if the receiver stops receiving.
+type naiveMerge struct {
+	subs    []Subscription
+	updates chan Item
+}
+
+// STARTNAIVEMERGE OMIT
+func NaiveMerge(subs ...Subscription) Subscription {
+	m := &naiveMerge{
+		subs:    subs,
+		updates: make(chan Item),
+	}
+	// STARTNAIVEMERGELOOP OMIT
+	for _, sub := range subs {
+		go func(s Subscription) {
+			for it := range s.Updates() {
+				m.updates <- it // HL
+			}
+		}(sub)
+	}
+	// STOPNAIVEMERGELOOP OMIT
+	return m
+}
+
+// STOPNAIVEMERGE OMIT
+
+// STARTNAIVEMERGECLOSE OMIT
+func (m *naiveMerge) Close() (err error) {
+	for _, sub := range m.subs {
+		if e := sub.Close(); err == nil && e != nil {
+			err = e
+		}
+	}
+	close(m.updates) // HL
+	return
+}
+
+// STOPNAIVEMERGECLOSE OMIT
+
+func (m *naiveMerge) Updates() <-chan Item {
+	return m.updates
+}
+
+type merge struct {
+	subs    []Subscription
+	updates chan Item
+	quit    chan struct{}
+	errs    chan error
+}
+
+// STARTMERGESIG OMIT
+// Merge returns a Subscription that merges the item streams from subs.
+// Closing the merged subscription closes subs.
+func Merge(subs ...Subscription) Subscription {
+	// STOPMERGESIG OMIT
+	m := &merge{
+		subs:    subs,
+		updates: make(chan Item),
+		quit:    make(chan struct{}),
+		errs:    make(chan error),
+	}
+	// STARTMERGE OMIT
+	for _, sub := range subs {
+		go func(s Subscription) {
+			for {
+				var it Item
+				select {
+				case it = <-s.Updates():
+				case <-m.quit: // HL
+					m.errs <- s.Close() // HL
+					return              // HL
+				}
+				select {
+				case m.updates <- it:
+				case <-m.quit: // HL
+					m.errs <- s.Close() // HL
+					return              // HL
+				}
+			}
+		}(sub)
+	}
+	// STOPMERGE OMIT
+	return m
+}
+
+func (m *merge) Updates() <-chan Item {
+	return m.updates
+}
+
+// STARTMERGECLOSE OMIT
+func (m *merge) Close() (err error) {
+	close(m.quit) // HL
+	for _ = range m.subs {
+		if e := <-m.errs; e != nil { // HL
+			err = e
+		}
+	}
+	close(m.updates) // HL
+	return
+}
+
+// STOPMERGECLOSE OMIT
+
+// NaiveDedupe converts a stream of Items that may contain duplicates
+// into one that doesn't.
+func NaiveDedupe(in <-chan Item) <-chan Item {
+	out := make(chan Item)
+	go func() {
+		seen := make(map[string]bool)
+		for it := range in {
+			if !seen[it.GUID] {
+				// BUG: this send blocks if the
+				// receiver closes the Subscription
+				// and stops receiving.
+				out <- it // HL
+				seen[it.GUID] = true
+			}
+		}
+		close(out)
+	}()
+	return out
+}
+
+type deduper struct {
+	s       Subscription
+	updates chan Item
+	closing chan chan error
+}
+
+// Dedupe converts a Subscription that may send duplicate Items into
+// one that doesn't.
+func Dedupe(s Subscription) Subscription {
+	d := &deduper{
+		s:       s,
+		updates: make(chan Item),
+		closing: make(chan chan error),
+	}
+	go d.loop()
+	return d
+}
+
+func (d *deduper) loop() {
+	in := d.s.Updates() // enable receive
+	var pending Item
+	var out chan Item // disable send
+	seen := make(map[string]bool)
+	for {
+		select {
+		case it := <-in:
+			if !seen[it.GUID] {
+				pending = it
+				in = nil        // disable receive
+				out = d.updates // enable send
+				seen[it.GUID] = true
+			}
+		case out <- pending:
+			in = d.s.Updates() // enable receive
+			out = nil          // disable send
+		case errc := <-d.closing:
+			err := d.s.Close()
+			errc <- err
+			close(d.updates)
+			return
+		}
+	}
+}
+
+func (d *deduper) Close() error {
+	errc := make(chan error)
+	d.closing <- errc
+	return <-errc
+}
+
+func (d *deduper) Updates() <-chan Item {
+	return d.updates
+}
+
+// FakeFetch causes Fetch to use a fake fetcher instead of the real
+// one.
+var FakeFetch bool
+
+// Fetch returns a Fetcher for Items from domain.
+func Fetch(domain string) Fetcher {
+	if FakeFetch {
+		return fakeFetch(domain)
+	}
+	return realFetch(domain)
+}
+
+func fakeFetch(domain string) Fetcher {
+	return &fakeFetcher{channel: domain}
+}
+
+type fakeFetcher struct {
+	channel string
+	items   []Item
+}
+
+// FakeDuplicates causes the fake fetcher to return duplicate items.
+var FakeDuplicates bool
+
+func (f *fakeFetcher) Fetch() (items []Item, next time.Time, err error) {
+	now := time.Now()
+	next = now.Add(time.Duration(rand.Intn(5)) * 500 * time.Millisecond)
+	item := Item{
+		Channel: f.channel,
+		Title:   fmt.Sprintf("Item %d", len(f.items)),
+	}
+	item.GUID = item.Channel + "/" + item.Title
+	f.items = append(f.items, item)
+	if FakeDuplicates {
+		items = f.items
+	} else {
+		items = []Item{item}
+	}
+	return
+}
+
+// realFetch returns a fetcher for the specified blogger domain.
+func realFetch(domain string) Fetcher {
+	return NewFetcher(fmt.Sprintf("http://%s/feeds/posts/default?alt=rss", domain))
+}
+
+type fetcher struct {
+	uri   string
+	feed  *rss.Feed
+	items []Item
+}
+
+// NewFetcher returns a Fetcher for uri.
+func NewFetcher(uri string) Fetcher {
+	f := &fetcher{
+		uri: uri,
+	}
+	newChans := func(feed *rss.Feed, chans []*rss.Channel) {}
+	newItems := func(feed *rss.Feed, ch *rss.Channel, items []*rss.Item) {
+		for _, it := range items {
+			f.items = append(f.items, Item{
+				Channel: ch.Title,
+				GUID:    it.Guid,
+				Title:   it.Title,
+			})
+		}
+	}
+	f.feed = rss.New(1 /*minimum interval in minutes*/, true /*respect limit*/, newChans, newItems)
+	return f
+}
+
+func (f *fetcher) Fetch() (items []Item, next time.Time, err error) {
+	fmt.Println("fetching", f.uri)
+	if err = f.feed.Fetch(f.uri, nil); err != nil {
+		return
+	}
+	items = f.items
+	f.items = nil
+	next = time.Now().Add(time.Duration(f.feed.SecondsTillUpdate()) * time.Second)
+	return
+}
+
+// TODO: in a longer talk: move the Subscribe function onto a Reader type, to
+// support dynamically adding and removing Subscriptions.  Reader should dedupe.
+
+// TODO: in a longer talk: make successive Subscribe calls for the same uri
+// share the same underlying Subscription, but provide duplicate streams.
+
+func init() {
+	rand.Seed(time.Now().UnixNano())
+}
+
+// STARTMAIN OMIT
+func main() {
+	// STARTMERGECALL OMIT
+	// Subscribe to some feeds, and create a merged update stream.
+	merged := Merge(
+		Subscribe(Fetch("blog.golang.org")),
+		Subscribe(Fetch("googleblog.blogspot.com")),
+		Subscribe(Fetch("googledevelopers.blogspot.com")))
+	// STOPMERGECALL OMIT
+
+	// Close the subscriptions after some time.
+	time.AfterFunc(3*time.Second, func() {
+		fmt.Println("closed:", merged.Close())
+	})
+
+	// Print the stream.
+	for it := range merged.Updates() {
+		fmt.Println(it.Channel, it.Title)
+	}
+
+	// Uncomment the panic below to dump the stack traces.  This
+	// will show several stacks for persistent HTTP connections
+	// created by the real RSS client.  To clean these up, we'll
+	// need to extend Fetcher with a Close method and plumb this
+	// through the RSS client implementation.
+	//
+	// panic("show me the stacks")
+}
+
+// STOPMAIN OMIT
diff --git a/concurrency/waitgroup/main.go b/concurrency/waitgroup/main.go
new file mode 100644
index 0000000..8edd7a1
--- /dev/null
+++ b/concurrency/waitgroup/main.go
@@ -0,0 +1,28 @@
+package main
+
+import (
+	"fmt"
+	"sync"
+)
+
+var wg sync.WaitGroup
+
+func main() {
+
+	for i := 0; i <= 10; i++ {
+		go print(i)
+		wg.Add(1)
+	}
+	wg.Wait()
+}
+
+func looprint(count int) {
+	for i := 0; i <= count; i++ {
+		fmt.Println(i)
+	}
+}
+
+func print(i int) {
+	fmt.Println(i)
+	wg.Done()
+}