interface Source
  method next() as word
end interface

class Counter implements Source
  private word value
  private word[4] seen

  public method init(start as word)
    self.value = start
  end method

  public method next() as word
    self.value = self.value + 1
    return self.value
  end method

  public method twice() as Counter
    return self
  end method

  public method fill(n as word)
    var i as word
    for i = 0 to 3 step 1
      seen[i] = self.next()
    end for
    for i = 3 to 0 step -1
      if seen[i] = 0
        return
      else
        if i > n and not (i = 2)
          seen[i] = seen[i - 1]
        end if
      end if
    end for
    while value < n
      value = value + 1
    end while
  end method
end class

program Main
  var c as Counter
  var s as Source
  var w as word
  c = new Counter((1 + 2) * 3)
  c.fill(c.next())
  c.twice().twice().fill(w)
  s = c
  w = s.next()
  print "done"
  print 'x'
  print true
  print w
  if s <> null
    delete s
  end if
end program