PL0-GNU-M2/synerr1.pl0

VAR i   : INTEGER;
var a,b : BOOLEAN;

PROCEDURE reduce;
BEGIN
   IF a > b THEN a := a - b;
   IF b > a THEN b := b - a;
END;

BEGIN
   ?a;
   ?b;
   WHILE  a#b DO CALL reduce;
   !a
END.