- Introduction

Nowadays, computer systems have become an important element of the world economy, and each year billions of dollars are spent to develop, manufacture, operate, and maintain various types of computer systems around the globe. Their reliability, safety, and usability have become an important concern because of problems such as high cost, wrong decisions and actions, and accidental deaths. For example, a study conducted by the National Institute of Standards and Technology in 2002 found that software defects alone cost the U.S. economy about $59 billion annually, i.e., around 0.6% of its gross domestic product (GDP) [1].