We will now construct a representation of the system based on abstract
data types (data encapsulation). To do this, we use a data structure
in the program as the basis for an abstract data type. We then select
all of the functions that use that data type and COLLAPSE them into a
SUBSYSTEM. The collapse operation adds another level to the hierarchy of
the subsystem decomposition. The new subsystem contains the data structure
and all of its access functions. First, we will create an abstract data type
for the "list" data structure.