let mut predicate = Predicate::new_trivial();

            // assign TRUE for puts and gets

            let true_ekeys = puts.keys().chain(gets.iter()).copied();

            let true_channel_ids = true_ekeys.clone().map(ekey_to_channel_id);

            predicate.batch_assign_nones(true_channel_ids, true);

            // assign FALSE for all in interface not assigned true

            predicate.batch_assign_nones(all_channel_ids.clone(), false);

            if branches.contains_key(&predicate) {

                // TODO what do I do with redundant predicates?

                unimplemented!(

                    "Having multiple batches with the same

                    predicate requires the support of oracle boolean variables"

                )

            }

            let branch = BranchN { to_get: gets, gotten: Default::default(), sync_batch_index };

            for (ekey, payload) in puts {

                log!(

                    &mut self.inner.logger,

                    "... ... Initial native put msg {:?} pred {:?} batch {:?}",

                    &payload,

                    &predicate,

                    sync_batch_index,

                );

                let msg =

                    CommMsgContents::SendPayload { payload_predicate: predicate.clone(), payload }

                        .into_msg(*round_index);

                endpoint_exts.get_mut(ekey).unwrap().endpoint.send(msg)?;

            }

            log!(

                &mut self.inner.logger,

                "... Initial native branch batch index={} with pred {:?}",

                sync_batch_index,

                &predicate

            );

            if branch.to_get.is_empty() {

                self.ephemeral.solution_storage.submit_and_digest_subtree_solution(

                    &mut self.inner.logger,

                    SubtreeId::PolyN,

                    predicate.clone(),

                );

            }

            branches.insert(predicate, branch);

        }

        Ok(PolyN { ekeys, branches })

    }

    // Runs a synchronous round until all the actors are in decided state OR 1+ are inconsistent.

    // If a native requires setting up, arg `sync_batches` is Some, and those are used as the sync batches.

        &mut self,

        mut deadline: Option<Instant>,

        sync_batches: Option<impl Iterator<Item = SyncBatch>>,

    ) -> Result<(), SyncErr> {

        log!(

            &mut self.inner.logger,

            "~~~~~~~~ SYNC ROUND STARTS! ROUND={} ~~~~~~~~~",

            self.inner.round_index

        );

        assert!(self.ephemeral.is_clear());

        // 1. Run the Mono for each Mono actor (stored in `self.mono_ps`).

        //    Some actors are dropped. some new actors are created.

        //    Ultimately, we have 0 Mono actors and a list of unnamed sync_actors

        self.ephemeral.mono_ps.extend(self.inner.mono_ps.iter().cloned());

        log!(&mut self.inner.logger, "Got {} MonoP's to run!", self.ephemeral.mono_ps.len());

        while let Some(mut mono_p) = self.ephemeral.mono_ps.pop() {

            let mut m_ctx = MonoPContext {

                ekeys: &mut mono_p.ekeys,

                mono_ps: &mut self.ephemeral.mono_ps,

                inner: &mut self.inner,

            };

            // cross boundary into crate::protocol

            let blocker = mono_p.state.pre_sync_run(&mut m_ctx, &self.protocol_description);

            log!(&mut self.inner.logger, "... MonoP's pre_sync_run got blocker {:?}", &blocker);

            match blocker {

                MonoBlocker::Inconsistent => return Err(SyncErr::Inconsistent),

                MonoBlocker::ComponentExit => drop(mono_p),

                MonoBlocker::SyncBlockStart => self.ephemeral.poly_ps.push(mono_p.into()),

            }

        }

        log!(

            &mut self.inner.logger,

            "Finished running all MonoPs! Have {} PolyPs waiting",

            self.ephemeral.poly_ps.len()

        );

        // 3. define the mapping from ekey -> actor

        //    this is needed during the event loop to determine which actor

        //    should receive the incoming message.

        //    TODO: store and update this mapping rather than rebuilding it each round.

        let ekey_to_holder: HashMap<Key, PolyId> = {

            use PolyId::*;

            let n = self.inner.mono_n.ekeys.iter().map(move |&e| (e, N));

            let p = self

                .ephemeral

                .poly_ps

                .iter()

                .enumerate()

                .flat_map(|(index, m)| m.ekeys.iter().map(move |&e| (e, P { index })));

            n.chain(p).collect()

        };

        log!(

            &mut self.inner.logger,

            "SET OF PolyPs and MonoPs final! ekey lookup map is {:?}",

            &ekey_to_holder

        );