The Structure of the "THE"-Multiprogramming System
A multiprogramming system is described in
which all activities are divided over a number of 
sequential processes.  These sequential processes are placed
at various hierarchical levels, in each 
of which one or more independent abstractions have been
implemented.  The hierarchical structure proved 
to be vital for the verification of the logical soundness
of the design and the correctness of its implementation.
CACM May, 1968
Dijkstra, E. W.
