diff --git a/prompts/EXISTING_REPO_CHECKLIST.md b/prompts/EXISTING_REPO_CHECKLIST.md
index f62e99a..fbc2214 100644
--- a/prompts/EXISTING_REPO_CHECKLIST.md
+++ b/prompts/EXISTING_REPO_CHECKLIST.md
@@ -214,15 +214,18 @@ with your task.
 - [ ] `script/install` and `script/update-policy` exist, identical to the model
       scripts, and `make update-policy` shims to `script/update-policy`.
       `make update-policy` commits without pushing, so a person reviews and
-      pushes the commit. Before its first use, the repo's own entries in
-      `.gitignore`, `.editorconfig` and `.dockerignore` go below the comment
-      that ends each canonical file. An entry above that comment which the
-      canonical file has never held makes the script stop before it writes
-      anything, naming the file and the entry. It stops the same way on a line
-      in `.golangci.yml` that the canonical file has never held, apart from
-      comments and the repo's entries in the `deny` list of the `test-support`
-      rule. It also stops on a repo's own `script/install` or
-      `script/update-policy`.
+      pushes the commit. A `.gitignore` or `.dockerignore` without the comment
+      that ends the canonical file needs no preparation. The script moves each
+      line the canonical file has never held to below the canonical content.
+      Blank lines and comments are dropped. Before its first use, add that
+      comment to an `.editorconfig` that has lines of its own, and put those
+      lines below it. Otherwise the script stops before it writes anything,
+      naming the file and each line. In a file that has the comment, an entry
+      above it which the canonical file has never held also stops the script. It
+      stops the same way on a line in `.golangci.yml` that the canonical file
+      has never held, apart from comments and the repo's entries in the `deny`
+      list of the `test-support` rule. It also stops on a repo's own
+      `script/install` or `script/update-policy`.
 - [ ] README has an **Entrypoints** section documenting the `script/`
       entrypoints and linking the standard
 - [ ] `script/lint` and `script/test` each run
diff --git a/script/install b/script/install
index 331be19..842f727 100755
--- a/script/install
+++ b/script/install
@@ -15,6 +15,12 @@
 #   - .gitignore, .editorconfig, .dockerignore: the lines below the
 #     canonical file's last line. An entry above that line which the
 #     canonical file has never held stops the script.
+#   - .gitignore, .dockerignore lacking that last line: each line the
+#     canonical file has never held, unless it is blank or a comment,
+#     goes below the canonical content.
+#   - .editorconfig lacking that last line: it is replaced if it has no
+#     such line. Otherwise the script stops, because a setting means
+#     nothing away from its section.
 #   - .golangci.yml: the entries the repo added to the deny list of the
 #     test-support rule. Any other line the canonical file has never
 #     held stops the script, unless it is blank or a comment.
@@ -117,12 +123,23 @@ local_only_lines() {
 # keep_lines_below_last <fetched> <local>: append to the fetched file
 # whatever follows its last line in the local file. Stop if the canonical
 # file has never held a line above that one, unless it is blank or a
-# comment: replacing the file would delete it.
+# comment: replacing the file would delete it. If the local file lacks
+# that last line, append each line the canonical file has never held,
+# unless it is blank or a comment. An .editorconfig with such a line
+# stops the script instead, because a setting means nothing away from
+# its section.
 keep_lines_below_last() {
     last="$(tail -n 1 "$1")"
     number="$(grep -n -F -x -- "$last" "$2" | head -n 1 | cut -d: -f1)"
     if [ -z "$last" ] || [ -z "$number" ]; then
-        die "$2 lacks this line; add it and put the repo's own entries below it: $last"
+        own="$(local_only_lines "$1" "$2")"
+        [ -n "$own" ] || return 0
+        if [ "$2" = .editorconfig ]; then
+            die "$2 lacks this line and has lines the canonical file has never held; add it and put those lines below it: $last
+$own"
+        fi
+        printf '%s\n' "$own" >> "$1"
+        return 0
     fi
     own="$(local_only_lines "$1" "$2" "$number")"
     if [ -n "$own" ]; then
diff --git a/test/update-policy.sh b/test/update-policy.sh
index 8a1ac08..4c1c144 100755
--- a/test/update-policy.sh
+++ b/test/update-policy.sh
@@ -45,7 +45,7 @@ not_found() {
 
 serve_file() {
     [ -f "$canonical/$1" ] || not_found
-    cp "$canonical/$1" "$output"
+    cat "$canonical/$1" > "$output"
 }
 
 case "$url" in
@@ -703,12 +703,37 @@ test_own_script_stops() {
 }
 
 test_gitignore_lacks_canonical_last_line() {
-    start_scenario ".gitignore lacking the canonical last line stops"
-    printf '*.log\n/myapp\n' > "$dir/repo/.gitignore"
+    start_scenario ".gitignore lacking the canonical last line keeps the repo's own line below the canonical content"
+    printf '*.log\n\n# A comment of our own.\n/myapp\n' > "$dir/repo/.gitignore"
     commit_and_push_fixture .gitignore .prettierrc
     run_update_policy
+    expect_success
+    expect_file .gitignore << 'EOF'
+*.log
+*.tmp
+
+# Own entries go below this comment.
+/myapp
+EOF
+    expect_new_commit_not_pushed
+}
+
+test_dockerignore_lacks_canonical_last_line() {
+    start_scenario ".dockerignore lacking the canonical last line drops a line an earlier canonical commit held"
+    echo "**/*.log" > "$dir/repo/.dockerignore"
+    expect_former_canonical_line_replaced .dockerignore ".git"
+    expect_same_as_canonical .dockerignore
+}
+
+test_editorconfig_lacks_canonical_last_line() {
+    start_scenario ".editorconfig lacking the canonical last line with a line of its own stops"
+    printf 'root = true\n[*.py]\nindent_size = 2\n' > "$dir/repo/.editorconfig"
+    commit_and_push_fixture .editorconfig .prettierrc
+    run_update_policy
     expect_failure
-    expect_output ".gitignore lacks this line"
+    expect_output ".editorconfig lacks this line"
+    expect_output "[*.py]"
+    expect_output "indent_size = 2"
     expect_nothing_written
 }
 
@@ -861,6 +886,8 @@ main() {
     test_ignored_file_stops
     test_own_script_stops
     test_gitignore_lacks_canonical_last_line
+    test_dockerignore_lacks_canonical_last_line
+    test_editorconfig_lacks_canonical_last_line
     test_fetched_golangci_without_deny_list
     test_own_golangci_setting_stops
     test_source_repo_stops