Refuse own-family reads before any survey, and panic on a clash where readSurvey.edge enters a read's pins

data.go

@@ -257,6 +257,9 @@ func checkNodeFences(s nodeScope) error {
 		return err
 	}
 	if refs {
+		if err := s(checkOwnFamilies); err != nil {
+			return err
+		}
 		if err := s(fence.checkDraws); err != nil {
 			return err
 		}

drawfence.go

@@ -24,7 +24,7 @@ func (f *drawFence) checkDrawGroup(path string, n node) error {
 	return nil
 }
 
-func (f *drawFence) checkDraws(path string, n node) error {
+func checkOwnFamilies(path string, n node) error {
 	t, ok := n.(*template)
 	if !ok {
 		return nil
@@ -32,6 +32,14 @@ func (f *drawFence) checkDraws(path string, n node) error {
 	if err := checkOwnFamily(t); err != nil {
 		return fmt.Errorf("%s: %w", path, err)
 	}
+	return nil
+}
+
+func (f *drawFence) checkDraws(path string, n node) error {
+	t, ok := n.(*template)
+	if !ok {
+		return nil
+	}
 	if !f.readsPath(t) {
 		return nil
 	}

readsurvey.go

@@ -122,7 +122,12 @@ func (s *readSurvey) edge(from node, e renderEdge, at surveyAt) {
 			s.reads = append(s.reads, pathRead{at, a, tr})
 		}
 		if tr != nil {
-			tr.pins.each(func(t *table, r int) { at.pins = at.pins.entered(t, r) })
+			tr.pins.each(func(t *table, r int) {
+				if c := at.pins.clash(t, r); c != nil {
+					panic(internalError("%s reads %s past a pinned row of %s, which checkOwnFamily refuses", e.reached(), t.selectorSpelling(r), c.path))
+				}
+				at.pins = at.pins.entered(t, r)
+			})
 		}
 	}
 	s.walk(e.to, at)