An expr entry replaces the previous three entries. It describes the sum in a single entry. The description names the register in which the sum can be found at this point in a running program. This maintainence of register numbers is the "glue" that connects compile time information and run time information. We can't know the value of the sum, but we can know where the code will leave it at run time.

We are now ready to perform the actual assignment.

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