BEGIN code1 flag WHILE code2 REPEAT
code1 is executed and flag is computed. If it is true,
code2 is executed and the loop is restarted; If flag is
false, execution continues after the REPEAT.
BEGIN code flag UNTIL
code is executed. The loop is restarted if flag is false.
Programming style note:
To keep the code understandable, a complete iteration of the loop should not change the number and types of the items on the stacks.
BEGIN code AGAIN
This is an endless loop.