Update property of a node

Is it possible to update the value of a node with a command or action ?
below an illustration about what I mean.

'stock': Number 'units'
'mutate': number 'units'
'mutation': stategroup (
  '+' { }
  '-' { }
'stock': number 'units' = swich .'mutation' (
  |'+' => sum ( .'stock', .'mutate' )
  |'-' => sum ( .'stock', - .'mutate' )

To update properties of a node using actions (or commands), you could do something like this:

	'stock items': collection ['item'] {
		'item': text
		'stock': number 'units'
		'mutate': action {
			'mutation': number 'units'
		} => update (
			'stock' = sum (
				@ .'mutation'

I hope this answers your question, because “update the value of a node” is unclear to me. Properties of a node can be updated. E.g. texts and numbers, but not the node itself.

Thanks for the correction, i did mean “update the value property of a node”
This indeed answered my question, thanks