public function createNamespace($namespaceName)
    {
        $unquotedNamespaceName = strtolower($this->getUnquotedAssetName($namespaceName));

        if (isset($this->namespaces[$unquotedNamespaceName])) {
            throw SchemaException::namespaceAlreadyExists($unquotedNamespaceName);
        }

        $this->namespaces[$unquotedNamespaceName] = $namespaceName;

        return $this;
    }