A node value is a value that is retrieved from a node and can only be deref'd when after the value as been provided by an action.
The action may not be executed on the same pallet machine as the phase, so the actual value is in the session map, and the node-value contains a key into the session map's :node-values key.
Positional factory function for class pallet.node_value.NodeValue.
A protocol used to read node-values
Throw an exception on access other than deref
Create an empty node-value
Return a node value symbol