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	diff --git a/memo/memo.go b/memo/memo.go
	index 8592b17..a31c8be 100644
	--- a/memo/memo.go
	+++ b/memo/memo.go
	@@ -1,56 +1,69 @@
	package memo

	import (
	"sync"
	+ "time"
	)

	type (
	Memo struct {
	f Func
	mu sync.Mutex //guards cache
	cache *entry
	+
	+ cacheDur time.Duration
	+ lastCache time.Time
	}

	Func func() (interface{}, error)
	)

	type (
	entry struct {
	res result
	ready chan struct{} // close when res is read
	}

	result struct {
	value interface{}
	err error
	}
	)

	-func New(f Func) *Memo {
	+func New(f Func, cd time.Duration) *Memo {
	return &Memo{
	- f: f,
	+ f: f,
	+ cacheDur: cd,
	}
	}

	-func (memo *Memo) Invalidate() {
	+// Invalidate resets the cache to nil if the memo's given cache duration has
	+// elapsed, and returns true if the cache was actually invalidated.
	+func (memo *Memo) Invalidate() bool {
	+ defer memo.mu.Unlock()
	memo.mu.Lock()
	- memo.cache = nil
	- memo.mu.Unlock()
	+
	+ if memo.cache != nil && time.Now().Sub(memo.lastCache) > memo.cacheDur {
	+ memo.cache = nil
	+ memo.lastCache = time.Now()
	+ return true
	+ }
	+ return false
	}

	func (memo *Memo) Get() (value interface{}, err error) {
	memo.mu.Lock()
	if memo.cache == nil {
	memo.cache = &entry{ready: make(chan struct{})}
	memo.mu.Unlock()

	memo.cache.res.value, memo.cache.res.err = memo.f()

	close(memo.cache.ready)
	} else {
	memo.mu.Unlock()

	<-memo.cache.ready
	}
	return memo.cache.res.value, memo.cache.res.err
	}

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