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.