Explain descendants removed by sequence replacement

facade_sequence_test.go

@@ -71,6 +71,37 @@ func TestFacadeItemHelperHistoryPreservesTraversalOrder(t *testing.T) {
 	}
 }
 
+func TestFacadeRegisteredReplacementExplainsRemovedDescendants(t *testing.T) {
+	t.Parallel()
+	for _, test := range []struct {
+		name  string
+		upper []any
+		path  configkit.Path
+	}{
+		{"shorter", []any{map[string]any{"id": "main"}}, configkit.Keys("records").Index(1).Key("id")},
+		{"empty", []any{}, configkit.Keys("records").Index(0).Key("id")},
+		{"removed field", []any{map[string]any{"id": "main"}}, configkit.Keys("records").Index(0).Key("text")},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			result, err := configkit.LoadLayered[map[string]any](configkit.Input{Source: configkit.Mapping("main", map[string]any{"records": test.upper})}, configkit.LayeredOptions[map[string]any]{
+				Definition: configkit.Definition[map[string]any]{SequenceCompositions: []configkit.SequenceComposition{{Path: configkit.Keys("records"), IdentityField: "id"}}},
+				Underlays:  []configkit.Underlay{{Inputs: []configkit.Input{{Source: configkit.Mapping("lower", map[string]any{"records": []any{map[string]any{"id": "one", "text": "old"}, map[string]any{"id": "two"}}})}}}},
+			})
+			if err != nil {
+				t.Fatal(err)
+			}
+			explanation, ok := result.Explain(test.path)
+			if !ok || explanation.Effective.Present || len(explanation.History) < 2 {
+				t.Fatalf("removal not explained: %#v", explanation)
+			}
+			last := explanation.History[len(explanation.History)-1]
+			if !last.Before.Present || last.After.Present || last.Origin.Layer != configkit.LayerBase || len(last.Origin.Locations) != 1 || last.Origin.Locations[0].SourceID != result.Sources()[0].ID {
+				t.Fatalf("wrong removal transition/origin: %#v", last)
+			}
+		})
+	}
+}
+
 func TestFacadeSequencePlacementControlsAcrossLoadEntryPoints(t *testing.T) {
 	definition := configkit.Definition[map[string]any]{
 		SequenceCompositions: []configkit.SequenceComposition{{Path: configkit.Keys("entries"), IdentityField: "id"}},