It’d be downward compatible – that is, a subset of Ruby that can run in all existing, current versions of Ruby (though not every version of Ruby could be run in Smaller Ruby). And explaining his rationale, Matsumoto puts up a slide that says “Smaller Ruby Can Run Faster (Hopefully).” He laughs as he reads it out loud, then adds “It’s just an idea. I don’t know yet. We have to keep compatibility… But if we stay in this kind of subset of Ruby…this is kind of inflexible, but much easier to optimize…”