HP 12C Buy vs rent decision program question

[acser]

In the HP 12c manual at http://h10032.www1.hp.com/ctg/Manual/c00363319.pdf on page 130, there is a sample program for the Buy vs Rent decision. I tried to key it and it works. Also tried to understand it, but I still cannot explain what program lines 48 and 49 do (the program is on page 131). If anyone can help, I would greatly appreciate it. I am also interested in other folks' interpretation of this program.

I created the stack diagram. Here it is: https://docs.google.com/spreadsheets/d/1...sp=sharing

Thanks in advance.