protected function setPrefix($prefix) {
        if ($this->configured) throw new IllegalStateException('Configuration is frozen');
        if ($len=strlen($prefix)) {
            if ($prefix[     0] == '/') throw new StrutsConfigException('Non-root module prefixes must not start with a slash "/" character, found: "'.$prefix.'"');
            if ($prefix[$len-1] != '/') throw new StrutsConfigException('Non-root module prefixes must end with a slash "/" character, found: "'.$prefix.'"');
        }
        $this->prefix = $prefix;
    }