Each event is processed immediately. The processing of non-reentrant events is followed by a succession of propagation turns until no more publishers are ready to emit, then a check for termination.

Ordering

Publisher ordering is derived from the lexicographical ordering of their ranks, where the rank of a publisher is the sequence of birth ranks of its successive parents (root first), stored in an array. The birth rank is computed by incrementing the field children of the parent on each publisher spawning.

Backpressure

Stream backpressure is tracked with the publisher field pending holding the number of subscriptions notified but not yet transferred, +1 if active this turn. It is incremented on emission and for each subscription notification, and decremented on end of active turn and for each subscription transfer. It is negative for signals (no backpressure).

Priority queues

A context schedules publishers ready to emit in two disjoint priority queues, for the current turn and the next one. Both queues are represented as the root of a pairing heap, or null if the queue is empty, respectively in today and tomorrow. Each publisher stores its first child in child and its next older sibling in sibling. When a publisher is removed from the heap, child is assigned to itself in order to efficiently check scheduling state.

Sets

Sets are needed for 3 purposes :

As long as a reactor is not cancelled, it maintains a set of cancellable publishers.
Between two successive emissions, a publisher maintains a set of notifiable subscriptions.
The set of emitting streams is maintained in order to deactivate them at the end of propagation turn.

1 & 2 represent the set as a doubly-linked list. The set manager keeps references to the first and last item in head and tail (both null if the set is empty), and the set items keep references to their predecessor and successor in prev and next (null respectively for the first and last items). When an item is removed, prev is assigned to itself in order to efficiently check set membership.

3 represents the set as a singly-linked list. The head of the list is stored in a local variable and the successor is stored in field active. When an item is inactive, active is assigned to itself.

# Events
A reactor context handles 7 kinds of events :
1. context boot
2. context cancellation
3. publisher notification
4. publisher termination
5. publisher cancellation
6. subscription transfer
7. subscription cancellation

Each event is processed immediately. The processing of non-reentrant events is followed by a succession of propagation
turns until no more publishers are ready to emit, then a check for termination.

# Ordering
Publisher ordering is derived from the lexicographical ordering of their ranks, where the `rank` of a publisher is the
sequence of birth ranks of its successive parents (root first), stored in an array. The birth rank is computed by
incrementing the field `children` of the parent on each publisher spawning.

# Backpressure
Stream backpressure is tracked with the publisher field `pending` holding the number of subscriptions notified but not
yet transferred, +1 if active this turn. It is incremented on emission and for each subscription notification, and
decremented on end of active turn and for each subscription transfer. It is negative for signals (no backpressure).

# Priority queues
A context schedules publishers ready to emit in two disjoint priority queues, for the current turn and the next one.
Both queues are represented as the root of a [pairing heap](https://www.cs.cmu.edu/~sleator/papers/pairing-heaps.pdf),
or `null` if the queue is empty, respectively in `today` and `tomorrow`. Each publisher stores its first child in
`child` and its next older sibling in `sibling`. When a publisher is removed from the heap, `child` is assigned to
itself in order to efficiently check scheduling state.

# Sets
Sets are needed for 3 purposes :
1. As long as a reactor is not cancelled, it maintains a set of cancellable publishers.
2. Between two successive emissions, a publisher maintains a set of notifiable subscriptions.
3. The set of emitting streams is maintained in order to deactivate them at the end of propagation turn.

1 & 2 represent the set as a doubly-linked list. The set manager keeps references to the first and last item in `head`
and `tail` (both `null` if the set is empty), and the set items keep references to their predecessor and successor in
`prev` and `next` (`null` respectively for the first and last items). When an item is removed, `prev` is assigned to
itself in order to efficiently check set membership.

3 represents the set as a singly-linked list. The head of the list is stored in a local variable and the successor is
stored in field `active`. When an item is inactive, `active` is assigned to itself.

source raw docstring