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It's a Smalltalk implementation written in Smalltalk, which is designed to survive its internals modified at runtime, which is the "live" part. The paper exhibits conventionally tricky bits to modify like the garbage collector and compiler, and describes some performance regressions they found which they fixed using the liveliness of the system.

I did indeed tilt my head to one side like a confused dog when I read that - this helped: https://en.m.wikipedia.org/wiki/Meta-circular_evaluator

https://www.google.com/search?q=Metacircular