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 question not 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]