public function send($consistency_level=null) {
        if ($consistency_level === null)
            $wcl = $this->cl;
        else
            $wcl = $consistency_level;

        $mutations = array();
        foreach ($this->buffer as $mut_set) {
            list($key, $cf, $cols) = $mut_set;

            if (isset($mutations[$key])) {
                $key_muts = $mutations[$key];
            } else {
                $key_muts = array();
            }

            if (isset($key_muts[$cf])) {
                $cf_muts = $key_muts[$cf];
            } else {
                $cf_muts = array();
            }

            $cf_muts = array_merge($cf_muts, $cols);
            $key_muts[$cf] = $cf_muts;
            $mutations[$key] = $key_muts;
        }

        if (!empty($mutations)) {
            $this->pool->call('batch_mutate', $mutations, $wcl);
        }
        $this->buffer = array();
    }