Reducer

Merges the states of nodes that ran in the same step and each returned a whole state.

Such nodes each produce their own copy of the state. The reducer takes the state before the step and those copies, and returns the one state the run continues with. Whatever it does not carry over from the copies is lost.

mergeRules builds a reducer from one rule for each property, such as "add to this list", so that nothing has to be carried over by hand.

Nodes added with a work and an update do not go through the reducer, and a graph whose parallel nodes are all of that kind needs none. See StateGraph.node and StateGraph.compile.