:f = "f"!script($_)
	:x = *{4}

	|x={x}

	:a = {1,2,3,(x):4}

	:c = a

	|1={a==c}

	|a(x)={a(x)}

	#ch a!checkpoint

	|a(x)={a(x)}

	#ch b!checkpoint

	x(1) = 3

	|a(x)={a(x)}

	#ch c!checkpoint

	|a(x)={a(x)}

	x={4}

	|no={a(x,())}

f!