sequence-diagram.puml

@startuml

' Number each step
autonumber
' Hide lower actor boxes
hide footbox
' Only greyscale
skinparam monochrome true
' Disable shadows
skinparam shadowing false

participant "Box A" as A
participant "Box B" as B
participant "Box C" as C

A -> B: Something
B -> C: More
C -> B: Return more
B -> A: Return something

@enduml