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. You can leave it by leaving the enclosing
colon definition with exit or throw, or with
while (see General loops with multiple exits).