Mermaid ZenUML Sequence Diagram with MAIDR

Left and right walk the messages in time, each with its route and the block it sits in; down moves onto each participant's lifeline.

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zenuml
  title Booking
  @Actor Customer
  @Boundary API
  @Database Store
  Customer->API.book(seat) {
    available = Store.check(seat)
    if(available) {
      Store.reserve(seat)
      return confirmation
    } else {
      return "sold out"
    }
  }
  API->Customer: receipt e-mailed