On the Capabilities of While, Repeat, and Exit Statements
A well-formed program is defined as a program
in which loops and if statements are properly 
nested and can be entered only at their beginning.  A
corresponding definition is given for a well-formed 
flowchart.  It is shown that a program is well formed if
and only if it can be written with if, repeat, 
and multi-level exit statements for sequence control. 
It is also shown that if,while, and repeat statements 
with single-level exit do not suffice.  It is also shown
that any flowcharts can be converted to a well-formed 
flowchart by node splitting.  Practical implications are discussed.
CACM August, 1973
Peterson, W. W.
Kasami, T.
Tokura, N.
