Defines the contract for sophisticated
safe feature extractions including assertion creation for the feature.
It is similar to NewFeatureAssertions but differs in the intended usage. NewFeatureAssertions are intended to make assertions about a return value of a method call or a property, assuming that the call as such always succeeds (no exception is thrown). The FeatureExtractor on the other hand should be used if it is already known, that the call/access fails depending on given arguments. For instance, List.get is a good example where it fails if the given index is out of bounds.