src/server/pkg/watch/watch.go
func (e *Event) UnmarshalPrev(key *string, val proto.Message) error {
	if err := CheckType(e.Template, val); err != nil {
		return err
	}
	*key = string(e.PrevKey)
	return proto.Unmarshal(e.PrevValue, val)
}
func (e *Event) UnmarshalPrev(key *string, val proto.Message) error {
	if err := CheckType(e.Template, val); err != nil {
		return err
	}
	*key = string(e.PrevKey)
	return proto.Unmarshal(e.PrevValue, val)
}
// UnmarshalPrev unmarshals the prev item in an event into a protobuf
// message.