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  • Mashrur Mia thinks this is interesting:

leaving the environment unchanged

From

Cover of Understanding Computation

Note

are we saying no side effect? cheeky

So far, we let our machine take an expression and an environment. Neither of them are changed. Internally,

self.expression = expression.reduce(enviroment)

is although called, we have made expression reduce return a new expression.

Environment is so far looked upon.