s1 = 'abcdabefgab'
      s2 = 'ab'
      s3 = 'xy'
      OUTPUT = ?(s1 ? POS(0) s2)  "1. " s2 " begins " s1
      OUTPUT = ?(s1 ? POS(0) s3)  "1. " s3 " begins " s1  ;# fails

      n = 0
again s1 POS(n) ARB s2 @a                        :F(p3)
      OUTPUT = "2. " s2 " found at position "
+        a - SIZE(s2) " in " s1
      n = a                                      :(again)

p3    OUTPUT = ?(s1 ? s2 RPOS(0)) "3. " s2 " ends " s1
END