tl;dr: When an "eval" redefines a top-level function, it does so AFTER the function returns. This makes it easier for the compiler to optimize appropriately. Th

tl;dr: When an "eval" redefines a top-level function, it does so AFTER the function returns. This makes it easier for the compiler to optimize appropriately. This is called "world age", where the compiler ensures that the age of a method is larger (newer) than the age of methods that are to be invoked. The rest of the paper formalizes this system into a logical calculus.