public function addBaselineCallable($name, $callable)
    {
        if (isset($this->callables[$name])) {
            throw new \InvalidArgumentException(sprintf(
                'Baseline callable "%s" has already been registered.',
                $name
            ));
        }

        if (!is_callable($callable)) {
            throw new \InvalidArgumentException(sprintf(
                'Given baseline "%s" callable "%s" is not callable.',
                $name, is_string($callable) ? $callable : gettype($callable)
            ));
        }

        $this->callables[$name] = $callable;
    }