It'd be useful to be able to draw connectors between shapes in different sub-diagrams.
The line c -> d in the code below currently creates 2 new participants, c and d, and draws a line between them.
But it'd be great if it could instead draw a connector between the 2 nodes c and d which already exist in sub-diagrams.

@startuml
a -> b
note left of a
{{
node c
}}
end note
/ note right of b
{{
node d
}}
end note

c -> d

@enduml