A polite librarian will nod and occasionally interject an umm, hmm, or really?! while suffering through a patron's recitation of his or her family genealogy. These proud orations can go on for hours if uninterrupted, so take precautions and situate yourself within reach of a fire alarm.
There is a reason these people hang out at the library all the time: Their families (which can be traced back "to George Washington's brother-in-law!") won't let these insufferable old birds back in the house.