One of the three types of software described by Lehman in his SPE program classification [LehmanBelady1985]. The distinctive properties of E-type systems are: the problem that they address cannot be formally and completely specified; the program has an imperfect model of the operational domain embedded in it; the program reflects an unbounded number of assumptions about the real world; the installation of the program changes the operation domain; the process of developing and evolving E-type system is driven by feedback.