Two groups on one card and a gap in between,
item1 holds still while item3 has to lean.
If the marker moves with it the binding is real —
if it stays where it sat, it was never a seal.

Assisted-by: Claude Opus 5 via Claude Code

diff --git a/tasks/probe.rake b/tasks/probe.rake
index e4f625d..6eea939 100644
--- a/tasks/probe.rake
+++ b/tasks/probe.rake
@@ -9,6 +9,47 @@
 # cooperation beyond running.
 
 namespace :probe do
+  # Puts a probe card where a client will sync it. The id is the UID:
+  # this server's cards are named by it (see ProTacts::Contact), and a
+  # PUT whose body disagrees is refused.
+  def seed(id, card)
+    require "net/http"
+    require "uri"
+
+    # rackup's default, which is what `rake dev` starts.
+    url = URI.parse(ENV.fetch("PROBE_URL", "http://localhost:9292"))
+    url.path = "/dav/addressbook/#{id}.vcf"
+
+    request = Net::HTTP::Put.new(url)
+    request["Content-Type"] = "text/vcard"
+    # Serve strips this header from incoming requests and sets it from
+    # the tailnet identity, so supplying one is only possible — and only
+    # meaningful — against a dev server on localhost
+    # (ProTacts::TailscaleAuth).
+    request["Tailscale-User-Login"] = ENV.fetch("PROBE_LOGIN", "probe@example.com")
+    request.body = card
+
+    response = Net::HTTP.start(url.hostname, url.port) { it.request(request) }
+    abort "#{url} answered #{response.code}: #{response.body}" unless response.code.start_with?("2")
+    response.code
+  end
+
+  # The shared half of every probe's instructions: a successful PUT is
+  # not an unhandled request, so the debug log is the only place it
+  # lands.
+  def reading_the_result(id, name)
+    <<~STEPS
+      1. The server needs PRO_TACTS_DEBUG=1 for the write to be logged —
+         a successful PUT is not an unhandled request, so log/unhandled
+         will not have it. Restart with `PRO_TACTS_DEBUG=1 rake dev` if
+         it is not set.
+      2. Resync the client, edit "#{name}" — any field, the edit itself
+         does not matter — and let the write go through.
+      3. The PUT body lands in log/dev.log after a `>>` line, under
+         #{id}.vcf.
+    STEPS
+  end
+
   # The annotation forms under test, one card so a single edit answers
   # all five. Each is independent: whatever the client keeps, drops, or
   # renumbers, it does to each of these on its own terms.
@@ -28,9 +69,6 @@ namespace :probe do
   #   item8.X-PT-GROUP      a companion beside an X-ABLabel that it
   #                         does, in case the known line is what
   #                         anchors the group through a renumber
-  #
-  # The id is the UID: this server's cards are named by it (see
-  # ProTacts::Contact), and a PUT whose body disagrees is refused.
   PROBE_ID = "probe-annotations"
   PROBE_CARD = <<~CARD.gsub("\n", "\r\n")
     BEGIN:VCARD
@@ -49,43 +87,76 @@ namespace :probe do
     END:VCARD
   CARD
 
+  # The follow-up to that one, which answered the companion question
+  # with numbering nothing here chose deliberately: item8 and item9,
+  # picked to dodge collisions, both got renumbered or stripped, so
+  # "the marker does not follow its group" and "the numbering confused
+  # the client" were not told apart.
+  #
+  # This card separates them. Two groups, both spelled the way a client
+  # spells its own — dense, in document order, each anchored by an
+  # X-ABLabel macOS models — with one gap:
+  #
+  #   item1  needs no renumbering. If its marker comes back at item1,
+  #          an untouched group keeps its companion.
+  #   item3  has to move to item2 for the numbering to be dense again.
+  #          Its marker either follows the group it belongs to, or it
+  #          stays at item3 while the group moves out from under it.
+  #
+  # The second is the whole experiment: a marker that stays put is a
+  # binding the client never maintained, and one that follows is a
+  # binding provenance could rest on. The first is the control that
+  # says the card was well-formed enough to be kept at all.
+  RENUMBER_ID = "probe-renumber"
+  RENUMBER_CARD = <<~CARD.gsub("\n", "\r\n")
+    BEGIN:VCARD
+    VERSION:3.0
+    N:Boole;Renumber;;;
+    FN:Probe Renumber
+    item1.TEL;TYPE=cell:+44 20 5555 0100
+    item1.X-ABLabel:household
+    item1.X-PT-GROUP:kxsv
+    item3.EMAIL;TYPE=internet:probe@example.com
+    item3.X-ABLabel:household
+    item3.X-PT-GROUP:mnop
+    UID:#{RENUMBER_ID}
+    END:VCARD
+  CARD
+
   desc "Seed the annotation probe card into a running dev server (PROBE_URL to override)"
   task :annotations do
-    require "net/http"
-    require "uri"
+    code = seed(PROBE_ID, PROBE_CARD)
 
-    # rackup's default, which is what `rake dev` starts.
-    url = URI.parse(ENV.fetch("PROBE_URL", "http://localhost:9292"))
-    url.path = "/dav/addressbook/#{PROBE_ID}.vcf"
+    puts <<~NEXT
+      Seeded #{PROBE_ID} (#{code}).
 
-    request = Net::HTTP::Put.new(url)
-    request["Content-Type"] = "text/vcard"
-    # Serve strips this header from incoming requests and sets it from
-    # the tailnet identity, so supplying one is only possible — and only
-    # meaningful — against a dev server on localhost
-    # (ProTacts::TailscaleAuth).
-    request["Tailscale-User-Login"] = ENV.fetch("PROBE_LOGIN", "probe@example.com")
-    request.body = PROBE_CARD
+      #{reading_the_result(PROBE_ID, "Probe Boole").chomp}
+      4. Compare it with the card in tasks/probe.rake: which of the five
+         forms came back, whether itemN was renumbered, and whether a
+         companion stayed with the line it was grouped with.
+      5. Record what happened in docs/macos-contacts.md. Repeat on iOS,
+         which numbers item groups differently (see
+         test/fixtures/ios-exchange/05-put-contact-edit/request).
+    NEXT
+  end
 
-    response = Net::HTTP.start(url.hostname, url.port) { it.request(request) }
-    abort "#{url} answered #{response.code}: #{response.body}" unless response.code.start_with?("2")
+  desc "Seed the renumbering probe: does a companion follow its group?"
+  task :renumber do
+    code = seed(RENUMBER_ID, RENUMBER_CARD)
 
     puts <<~NEXT
-      Seeded #{PROBE_ID} (#{response.code}).
+      Seeded #{RENUMBER_ID} (#{code}).
 
-      1. The server needs PRO_TACTS_DEBUG=1 for the write to be logged —
-         a successful PUT is not an unhandled request, so log/unhandled
-         will not have it. Restart with `PRO_TACTS_DEBUG=1 rake dev` if
-         it is not set.
-      2. Resync the client, edit "Probe Boole" — any field, the edit
-         itself does not matter — and let the write go through.
-      3. The PUT body lands in log/dev.log after a `>>` line. Compare it
-         with the card above: which of the five forms came back, whether
-         itemN was renumbered, and whether a companion stayed with the
-         line it was grouped with.
-      4. Record what happened in docs/macos-contacts.md. Repeat on iOS,
-         which numbers item groups differently (see
-         test/fixtures/ios-exchange/05-put-contact-edit/request).
+      #{reading_the_result(RENUMBER_ID, "Probe Renumber").chomp}
+      4. Read the two groups that come back:
+         - item1.TEL and its X-ABLabel should not have to move. Did
+           item1.X-PT-GROUP come back beside them?
+         - item3.EMAIL and its X-ABLabel should land on item2. Did
+           item3.X-PT-GROUP follow them there, or stay at item3?
+         The second answers whether a companion is a binding the client
+         maintains or two numbers that happened to agree.
+      5. Record what happened in docs/macos-contacts.md, under "An
+         annotation survives only on its own line".
     NEXT
   end
 end