Emulation is a strategy in digital preservation to combat obsolescence. The product life cycle concept consist of 4 stages&colon; introduction, growth, maturity, obsolescence. Planned obsolescence plays a key role in overconsumption. The dagger is also used to indicate death, extinction, or obsolescence. This variety of codecs can create compatibility and obsolescence issues. Open source software is often cited as solution for preventing digital obsolescence. The possible obsolescence of stealth technology was also criticized. Estimates of planned obsolescence can influence a company's decisions about product engineering. Formats proprietary to one software vendor are more likely to be affected by format obsolescence. This competitive advantage helped to keep USIA enterprises afloat despite their mounting obsolescence. A dozen of these devices were built before their obsolescence became obvious. This process can be referred to as digital obsolescence. These characteristics are used to derive an "annual obsolescence rate" and "environmental obsolescence". Likewise the Romans had no concept of obsolescence. As a stopgap, the 912E was the single instance of "planned obsolescence" in Porsche history.