self.0.push(!0u32);

            } else {

                if end > 0 {

                    // #end ones, with a (32-end) prefix of zeroes

                    self.0.push(!0u32 >> (32 - end));

                }

                return;

            }

        }

    }

    #[inline(always)]

    fn index_decomposed(index: usize) -> [usize; 2] {

        // [chunk_index, chunk_bit]

        [index / 32, index % 32]

    }

    fn test(&self, at: usize) -> bool {

        let [chunk_index, chunk_bit] = Self::index_decomposed(at);

        match self.0.get(chunk_index) {

            None => false,

            Some(&chunk) => (chunk & (1 << chunk_bit)) != 0,

        }

    }

    fn set(&mut self, at: usize) {

        let [chunk_index, chunk_bit] = Self::index_decomposed(at);

        if chunk_index >= self.0.len() {

            self.0.resize(chunk_index + 1, 0u32);

        }

        let chunk = unsafe {

            // SAFE! previous line ensures sufficient size

            self.0.get_unchecked_mut(chunk_index)

        };

        *chunk |= 1 << chunk_bit;

    }

    fn unset(&mut self, at: usize) {

        let [chunk_index, chunk_bit] = Self::index_decomposed(at);

        if chunk_index < self.0.len() {

            let chunk = unsafe {

                // SAFE! previous line ensures sufficient size

                self.0.get_unchecked_mut(chunk_index)

            };

            *chunk &= !(1 << chunk_bit);

            while let Some(0u32) = self.0.iter().copied().last() {

                self.0.pop();

            }

        }

    }

}

struct BitChunkIter<I: Iterator<Item = u32>> {

    chunk_iter: I,

    next_bit_index: usize,

    cached: Option<u32>, // None <=> iterator is done

}

impl<I: Iterator<Item = u32>> BitChunkIter<I> {

    fn new(mut chunk_iter: I) -> Self {

        let cached = chunk_iter.next();

        Self { chunk_iter, next_bit_index: 0, cached }

    }

}

impl<I: Iterator<Item = u32>> Iterator for BitChunkIter<I> {

    type Item = usize;

    fn next(&mut self) -> Option<Self::Item> {

        loop {

            println!("LOOP");

            // get cached chunk. If none exists, iterator is done.

            let mut chunk = self.cached?;

            if chunk == 0 {

                // self.next_bit_index jumps to next multiple of 32

                self.next_bit_index = (self.next_bit_index + 32) & !(32 - 1);

                self.cached = self.chunk_iter.next();

                continue;

            }

            // this chunk encodes 1+ Items to yield

            // shift the contents of chunk until the least significant bit is 1

            #[inline(always)]

            fn shifty(chunk: &mut u32, shift_by: usize, next_bit_index: &mut usize) {

                if *chunk & ((1 << shift_by) - 1) == 0 {

                    *next_bit_index += shift_by;

                    *chunk >>= shift_by;

                }

                println!("{:#032b}", *chunk);

            }

            shifty(&mut chunk, 16, &mut self.next_bit_index);

            shifty(&mut chunk, 08, &mut self.next_bit_index);

            shifty(&mut chunk, 04, &mut self.next_bit_index);

            shifty(&mut chunk, 02, &mut self.next_bit_index);

            shifty(&mut chunk, 01, &mut self.next_bit_index);

            // assert(chunk & 1 == 1)

            self.next_bit_index += 1;

            self.cached = Some(chunk >> 1);

            if chunk > 0 {

                return Some(self.next_bit_index - 1);

            }

        }

    }

}

/// Returns an iterator over chunks of bits where ALL of the given

/// bitsets have 1.

struct AndChunkIter<'a> {

    // this value is not overwritten during iteration

    // invariant: !sets.is_empty()

    sets: &'a [&'a [u32]],

    next_chunk_index: usize,

}

impl<'a> AndChunkIter<'a> {

    fn new(sets: &'a [&'a [u32]]) -> Self {

        let sets = if sets.is_empty() { &[&[] as &[_]] } else { sets };

        Self { sets, next_chunk_index: 0 }

    }

}

impl Iterator for AndChunkIter<'_> {

    type Item = u32;

    fn next(&mut self) -> Option<u32> {

        let old_chunk_index = self.next_chunk_index;

        self.next_chunk_index += 1;

        self.sets.iter().fold(Some(!0u32), move |a, b| {

            let a = a?;

            let b = *b.get(old_chunk_index)?;

            Some(a & b)

        })

    }

}

#[test]

fn test_bit_iter() {

    static SETS: &[&[u32]] = &[

        //

        &[0b101001, 0b101001],

        &[0b100001, 0b101001],

    ];

    let indices = iter.collect::<Vec<_>>();

    println!("indices {:?}", indices);

}

enum Entity {

    Payload(Payload),

    Machine { state: ProtocolS, component_index: usize },

}

struct PortKey(usize);

struct EntiKey(usize);

struct CompKey(usize);

struct ComponentInfo {

    port_keyset: HashSet<PortKey>,

    protocol: Arc<Protocol>,

}

#[derive(Default)]

struct Connection {

    ecs: Ecs,

    round_solution: Vec<(ChannelId, bool)>, // encodes an ASSIGNMENT

    ekey_channel_ids: Vec<ChannelId>,       // all channel Ids for local keys

    component_info: Vec<ComponentInfo>,

    endpoint_exts: Vec<EndpointExt>,

}

/// Invariant: every component is either:

///        in to_run = (to_run_r U to_run_w)

///     or in ONE of the ekeys (which means it is blocked by a get on that ekey)

///     or in sync_ended (because they reached the end of their sync block)

///     or in inconsistent (because they are inconsistent)

#[derive(Default)]

struct Ecs {

    entities: Vec<Entity>, // machines + payloads

    assignments: HashMap<(ChannelId, bool), BitSet>,

    payloads: BitSet,

    ekeys: HashMap<usize, BitSet>,

    inconsistent: BitSet,

    sync_ended: BitSet,

    to_run_r: BitSet, // read from and drained while...

    to_run_w: BitSet, // .. written to and populated. }

}

impl Debug for Ecs {

    fn fmt(&self, f: &mut Formatter) -> std::fmt::Result {

        let elen = self.entities.len();

        write!(f, "{:<30}", "payloads")?;

        print_flag_bits(f, &self.payloads, elen)?;