https://lviv.dotnet.city/2010/10/%d0%bf%d0%b0%d1%82%d0%b5%d1%80%d0%bd-%d0%b2%d1%96%d0%b2%d1%82%d0%be%d1%80%d0%ba%d0%b0-10-%d1%96%d1%82%d0%b5%d1%80%d0%b0%d1%82%d0%be%d1%80-iterator/