Simula put each of those data structures together with all its procedures in a tightly integrated package, so that each structure "knew" how to respond to commands. In practice, this meant that a Simula programmer could model an oil refinery, say, in much the same way that he or she thought about a real refinery: not as a list of abstract data structures and equally abstract procedures, but in terms of valves, pipes, tanks, and whatever—tangible objects that had well-defined properties and characteristic behaviors. The potential gain in conceptual clarity was enormous.
