private function getProvider($identifier)
    {
        if (isset($this->interfaceProviders[$identifier])) {
            $this->assertSingleInterfaceProvider($identifier);
            return reset($this->interfaceProviders[$identifier]);
        } elseif (isset($this->classProviders[$identifier])) {
            $this->assertSingleClassProvider($identifier);
            return reset($this->classProviders[$identifier]);
        } else {
            return false;
        }
    }