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Steele and Sussman
35
The Art of the Interpreter
variable. If there is no such binding, then the intent is to initialize a top-level variable; EV-TOP-LEVEL-SETQ locates the top-level environment (which is always at the end of any environment) and creates a new binding by altering the environment structure.
We also modify EVAL to recognize PROGN. EVPROGN is a tail-recursive loop which evaluates each subform of the PROGN form in turn, throwing away each value but the last. {Note PROGN Wizardry}
