A filter branch that ends at a sink.
Sink filters such as FFix.Filter.anullsink/2 consume a stream and return a
terminal. Include it in FFix.command(outputs, terminals: [terminal]) to
execute that branch, or store it in a graph with FFix.graph/1.
See FFix.Filter for branching and FFix.Graph.terminals/1 for retrieving
sinks from a reusable graph.