Enum Topology 

pub enum Topology {
    Leaf(Box<dyn FrameProcessor>),
    Chain(Vec<Topology>),
    Parallel(Vec<Topology>),
}

The wiring shape a processor expands to when a crate::pipeline::Pipeline links its members (PROCESSOR-DESIGN §3). A leaf is one spawned task; a Chain is wired head→tail in order; a Parallel fans one input into every branch and funnels/dedups the outputs. This is how Pipeline/ParallelPipeline nest without a downcast.

Variants

Leaf(Box<dyn FrameProcessor>)

A single processor → one spawned task.

Chain(Vec<Topology>)

A linear chain wired head→tail (a Pipeline’s members).

Parallel(Vec<Topology>)

A fan-out/fan-in block (a ParallelPipeline’s branches).