sub check_brackets(s$)
    local level, i
	
    for i = 1 to len(s$)
        switch mid$(s$, i, 1)
            case "[": level = level + 1 : break
            case "]": level = level - 1 : if level < 0 break 2
        end switch
    next i
    return level = 0
end sub

s$ = "[[]][]"

print s$, " = ";

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