We will find it helpful later if we organise our quantities into
the natural collections of interest. In our example our four quantities
are 
, the collection of quantities about which we wish
to learn; and 
, the collection of observable
quantities. Our term for collections like these is base, and
within [B/D] we define these structures by using the BASE:
command as follows:
BD>base:B = B1, B2 
BD>base:D = D1, D2
BD>base:G = B, D
Here we have specified the two natural bases, named ``B'' and ``D''
(there is no extra meaning attached to using the names B and D; we could
have used ``bacon'' and ``eggs'', or anything else that takes our fancy) and
a further base named ``G'' to contain all the quantities for our later
convenience. Notice that we defined this last base in terms of
previously defined bases
. The names that we use are subject to the same rules that
are used to name elements. We can check our definitions by issuing the
command
BD>look:(b)
The output, shown in figure 6, lists the names of the bases that [B/D] knows about, along with their contents. Notice that the component quantities of the bases are listed in alphabetical order. The ordering is important because we will frequently use a base name as shorthand for the collection of quantities that it represents, and we will need to know the order in which quantities within a base will be affected by our actions. Notice also that the base ``G'' contains other bases rather than elements.
