How to tweak syslinux to boot an iso image?

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http://askubuntu.com – Let me start saying I found lots of stuff googling but I haven't found nothing that could help me solving my particular issue. I made my Ubuntu persistent LiveUSB, which comes with syslinux to choose between "Try Ubuntu without installing", "Install Ubuntu", etc. I'd like to tweak the syslinux on the USB drive to have an option to boot another .iso image (which would probably be another ubuntu flavor). Bonus points: specify boot options for the .iso image, which comes with syslinux, too. Related questions:http://askubuntu.com/questions/155834/how-do-i-create-a-liveusb-that-lets-me-install-a (HowTos)