The parallel node

{
  "id": "fan_out",
  "type": "parallel",
  "branches": [{ "id": "hash", "entry": "hash_step" },
               { "id": "thumb", "entry": "thumb_step" }],
  "next": "merge"
}

Only names that were not in the frame before parallel leave a branch; a write to a name that existed on entry stays branch-local. Such names are listed in the parallel_completed event: {"merged": ["fresh"], "dropped": ["left.n"]}. Two branches writing the same name raise BranchError naming both.

cancel_on_error (default true) decides the fate of sibling branches when one fails: true cancels them immediately (the parallel_cancelled event), false lets them finish. Either way the node fails with the error of the first branch that failed.

The editor draws the branches as a framed region under the node: