code: func (r *PrevRevisions) Put(key string, val datasync.LazyValue) (
  	found bool, prev datasync.KeyVal, currRev int64) {
  
  	found, prev = r.Get(key)
  	if prev != nil {
  		currRev = prev.GetRevision() + 1
  	} else {
  		currRev = 0
  	}
  
  	r.mu.Lock()
  	r.revisions[key] = &valWithRev{
  		LazyValue: val,
  		key:       key,
  		rev:       currRev,
  	}
  	r.mu.Unlock()
  
  	return found, prev, currRev
  }
docs: // Put updates the entry in the revisions and returns previous value.