final public function add($capability, $specUrl, $specVersion) {

        if ( array_key_exists($capability, $this->rpc_capabilities) ) {

            $this->logger->warning("Cannot add capability $capability: duplicate entry");

            return false;

        } else {

            $this->rpc_capabilities[$capability] = array(
                'specUrl' => $specUrl,
                'specVersion' => $specVersion
            );

            $this->logger->debug("Added capability $capability");

            return true;

        }

    }