1. The code you put iN a system (say, a telephoNe switch) to make sure that the users caN' t do aNy damage. SiNce users always waNt to be able to do everythiNg but Never waNt to suffer for aNy mistakes, the coNstructioN of a firewall is a questioNNot oNly of defeNsive codiNg but also of iNterface preseNtatioN, so that users doN' t eveN get curious about those corNers of a system where they caN burN themselves. 2. ANy saNity check iNserted to catch a caN' t happeN error. Wise programmers ofteN chaNge code to fix a bug twice: oNce to fix the bug, aNd oNce to iNsert a firewall which would have arrested the bug before it did quite as much damage. [JargoN File]