Function causal 

pub fn causal<'b, 'a: 'b, E: Envelope<'a>>(
    envelopes: &'b mut Vec<E>,
    topic_cursor: &'b mut TopicCursor,
) -> impl Sort<Vec<E>> + use<'a, 'b, E>

Sorts Envelopes Causally in-place All envelopes part of Self will be sorted. Envelopes with missing dependencies will be returned. TopicCursor will be updated to reflect causally-verified envelopes. XIP Definition A causal sort orders envelopes by their dependencies. A dependency is defined as an envelope which must be processed before any dependant envelopes.

Unsorted (arrival order):        Sorted (causal order):

  [E3] ──depends on──> [E1]         [E1] ───→ [E2] ───→ [E3]
  [E1] (no deps)                     ↑         ↑
  [E2] ──depends on──> [E1]          │         │
                                  (no deps) (needs E1)

E1 must be processed first, then E2, then E3

Arguments

• envelopes: the Envelope’s being sorted
• topic_cursor: the cursor position of all known topics