ooad1-7  Exercise 2: Nightly Overdue Notices
================================================

evening.fragments.mmd (Mermaid 12.1.0):

  sequenceDiagram
      participant Timer as Nightly Timer
      participant Desk as LoanDesk
      participant Notifier
      Timer->>Desk: send_overdue_notices(today)
      loop for each overdue loan
          Desk-)Notifier: email(member, loan)
          opt the member has a mobile number
              Desk-)Notifier: sms(member, loan)
          end
      end
      Desk-->>Timer: number of notices sent

ex2.py (needs compare_diagrams.py and the class diagram source from the chapter folder):

  """Exercise 2: which operations does the new sequence diagram ask for that the class diagram does not have yet?
  Run from the chapter folder:  py ex2/ex2.py"""
  import pathlib
  import re
  import sys

  HERE = pathlib.Path(__file__).resolve().parent.parent
  sys.path.insert(0, str(HERE))
  from compare_diagrams import calls, class_operations  # noqa: E402

  source = pathlib.Path(__file__).parent / "evening.fragments.mmd"
  text = source.read_text(encoding="utf8")
  aliases = dict(re.findall(r"participant (\w+) as ([\w ]+)", text))

  ops = class_operations(HERE / "diagrams" / "version_b.class.mmd")
  print("operations the diagram asks for (receiver.operation):")
  for sender, receiver, op in sorted(calls(source)):
      cls = aliases.get(receiver, receiver)
      status = "present" if op in ops.get(cls, set()) else ("class missing" if cls not in ops else "operation missing")
      print(f"   {cls}.{op:<22} {status}")

Run with:  py ex2/ex2.py

Output:

  operations the diagram asks for (receiver.operation):
     Notifier.email                  class missing
     Notifier.sms                    class missing
     LoanDesk.send_overdue_notices   operation missing

What the diagram says: the timer asks the desk to send notices; for each overdue loan the desk sends an email without waiting (an open arrow, a fire-and-forget message); an opt box holds the text message, sent only if the member has a mobile number. The final return reports how many notices were sent.
Why loop around opt, not par: the email and text for one member are not required to happen at the same time; the question is whether the text happens at all, which is an opt. A par would be right if the two had to run concurrently and neither depended on the other.

What the check found: three decisions are waiting. The class diagram has no Notifier; Notifier needs email and sms operations; and LoanDesk needs send_overdue_notices(today). Also visible in the diagram but not checked by the script: the desk must be able to list overdue loans, which is another operation (probably on the Catalogue), and a Member needs a way to say whether it has a mobile number.

WHY THIS WORKS AS AN ANSWER
---------------------------
Each arrow is an operation promise, so a sequence diagram doubles as a to-do list for the class diagram. The check is crude (it matches names, and it cannot see operations hidden inside a loop's condition), but it found the gaps in seconds.