Notes on the sub-directories included in this directory

VPIM is described hierarchically in PSL.  From the top, the layers are:

	- interpreter, 
	- functions, 
	- structures, 
	- basics, 
	- primitives.  

Routines defined in each layer have similar granularity in terms of
function and abstraction.  A routine in one layer must not invoke
other routines in higher layers.
