Parallel node fan-out and fan-in with extra steps and Deletions

[NadimKawwaProphia]

Been looking through the docs and seems like there isn't anything for a deletions. All the examples assume we are editing or adding.

In the graph below we want the commence_date_review-->term_expiration_date_review to execute while pro_rata_share_review executes in parallel, and then merge the two results.

I figure more explanation is needed:

• commence_date_review will pop (remove) keys from the state dict
• term_expiration_date_review will add and/or edit key values in the state dict
• pro_rata_share_review will add/edit key values in the state dict.

[eyurtsev]

You can specify a custom reducer that has arbitrary logic (by specifying a custom function). The function takes as input the current value and the requested update to produce a new value. You can choose whatever representation you want for the update... e.g, it can be a 2-tuple w/ the first key indicating whether it's an append or delete operation.