requires_pure(&() p)
{
	p();
	/* ok since closure is pure */
	p();
}

main()
{
	20 => int twenty;
	requires_pure() &=> {
		/* Try uncommenting!
		 * Not allowed in pure context (though primitives should maybe
		 * be excluded just to make people's lives easier?)
		 */
		// twenty + 10 => int thirty;
	}
}