The description of some task situation may take the form of a large corpus of lists, representing propositions. For instance we might know that B1, B2, ... etc are blocks, that each has a colour and a size, and that some are on others. These facts could be represented as a database consisting of a list of lists:
[[b1 isa block] [size b1 big] [colour b1 green] [on b1 b2] ......... ]We could also describe the contents of an image in terms of which objects are in it what their properties are, and how they are related. The library program SEEPICTURE builds up just such a list representation of a PICTURE pattern to provide a basis for recognition.