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A hierarchy of control structures

Four classes of control structures for sequential programs are shown to offer increasing expressive power. They allow control to be specified by 
 
(1) 
 
four well-chosen structures with one entry and one exit each, 
 
 
 
 
(2) 
 
the one-entry loop with escapes, 
 
 
 
 
(3) 
 
the distributor-entered loop with escapes, and 
 
 
 
 
(4) 
 
the unrestricted jump. 
 
 
 
 
 
 
It is shown that a given program scheme can be described by the one-entry loop with escapes if and only if all control paths from the start to a cycle enter that cycle at the same node (irrespective of the number of nodes where the cycle is re-entered).