func (e *element) IsExpired(now time.Time) bool {
	return now.After(e.expiration)
}
IsExpired returns true if the element is expired (current time is greater
than the expiration time).
package common
import (
	"sync"
	"time"
)
type Key interface{}
type Value interface{}
type RemovalListener func(k Key, v Value)
type clock func() time.Time
type element struct {
	expiration time.Time
	timeout    time.Duration
	value      Value
}