s$ = "[[]][]"
print s$; " = ";

if not check_brackets(s$) then print "not ";
print "ok"
end

function check_brackets(s$)
  level = 0
  for i = 1 to length(s$)
    c$ = mid(s$, i, 1)
    begin case
      case c$ = "["
        level = level + 1
      case c$ = "]"
        level = level - 1
        if level < 0 then exit for
    end case
  next i
  return level = 0
end function