diff --git a/crates/weksa/src/split.rs b/crates/weksa/src/split.rs
index d22d3db..20f971d 100644
--- a/crates/weksa/src/split.rs
+++ b/crates/weksa/src/split.rs
@@ -172,6 +172,9 @@ impl Checkpoint {
     }
 }
 
+#[cfg(test)]
+mod mutants;
+
 /// Item builders for the unit tests that live in `src`: the twins of
 /// `tests/common`'s `group_in_bucket`, `item_in`, `cast` and `model`.
 #[cfg(test)]
diff --git a/crates/weksa/src/split/mutants.rs b/crates/weksa/src/split/mutants.rs
new file mode 100644
index 0000000..99cde9f
--- /dev/null
+++ b/crates/weksa/src/split/mutants.rs
@@ -0,0 +1,11 @@
+//! Pins of private items of `split` found unpinned by mutation testing.
+
+use super::Checkpoint;
+
+/// A checkpoint reports the round it was declared at, whatever the round.
+#[test]
+fn a_checkpoint_reports_its_round() {
+    assert_eq!(Checkpoint::for_test(7).round(), 7);
+    assert_eq!(Checkpoint::for_test(0).round(), 0);
+    assert_eq!(Checkpoint::for_test(1).round(), 1);
+}
diff --git a/crates/weksa/tests/mutants_a.rs b/crates/weksa/tests/mutants_a.rs
new file mode 100644
index 0000000..a9ddda5
--- /dev/null
+++ b/crates/weksa/tests/mutants_a.rs
@@ -0,0 +1,25 @@
+//! Pins for behaviour of the small and middle weksa files that mutation testing
+//! found unpinned. Each test states the behaviour in domain terms.
+
+use weksa::form::is_calendar_date;
+
+/// A calendar date is exactly `YYYY-MM-DD`: both hyphens sit where the format
+/// puts them, so a ten-byte string with a digit in the second hyphen's place is
+/// no date.
+#[test]
+fn a_calendar_date_needs_both_hyphens_in_place() {
+    assert!(is_calendar_date("2026-01-01"));
+    assert!(!is_calendar_date("2026-01201"));
+    assert!(!is_calendar_date("2026101-01"));
+}
+
+/// February has 29 days in a Gregorian leap year and 28 otherwise.
+#[test]
+fn february_has_29_days_only_in_gregorian_leap_years() {
+    assert!(is_calendar_date("2024-02-29"));
+    assert!(is_calendar_date("2000-02-29"));
+    assert!(!is_calendar_date("1900-02-29"));
+    assert!(!is_calendar_date("2023-02-29"));
+    assert!(is_calendar_date("2023-02-28"));
+    assert!(!is_calendar_date("2024-02-30"));
+}