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.