code: func (p *Pss) checkFwdCache(msg *PssMsg) bool {
  	p.fwdCacheMu.Lock()
  	defer p.fwdCacheMu.Unlock()
  
  	digest := p.digest(msg)
  	entry, ok := p.fwdCache[digest]
  	if ok {
  		if entry.expiresAt.After(time.Now()) {
  			log.Trace("unexpired cache", "digest", fmt.Sprintf("%x", digest))
  			metrics.GetOrRegisterCounter("pss.checkfwdcache.unexpired", nil).Inc(1)
  			return true
  		}
  		metrics.GetOrRegisterCounter("pss.checkfwdcache.expired", nil).Inc(1)
  	}
  	return false
  }
docs: // check if message is in the cache