ref

ref - a cell holding a value that a closure has to change.

A closure reads the variables of the function around it, but it cannot write to them: assigning to one of those names makes a local of its own, which starts from the value read on the right of the assignment and shadows the name from there on. So this counts to three and then stops counting:

mk = fn() {
	n = 0
	return fn() { n = n + 1; return n }   # always 1
}

What a closure can change is the inside of something it captured, because the name keeps meaning the same thing. A cell is that something:

mk = fn() {
	n = ref.New(0)
	return fn() { n.v = n.v + 1; return n.v }   # 1, 2, 3, ...
}

Two closures made in the same place share the cell, so one can write what the other reads. That also means a cell handed to a tau routine is shared with it and nothing guards it: for values that travel between routines use a pipe, which is what pipes are for.