State the rule each draw fence enforces beside it: the cell drop in walk, alternatives, replayPairs and heldCheck

drawfence.go

@@ -225,7 +225,8 @@ func newDrawWalk() *drawWalk {
 }
 
 // walk follows what rendering n renders. A repeat renders over draws of its own, so the walk stops
-// there.
+// there. A cell whose row the pins keep out has no case: a pinned table renders its pinned row
+// alone, so that cell never renders on this route.
 func (w *drawWalk) walk(n node, at drawAt) {
 	v := nodeVisit{n, at.group, at.rowsKey()}
 	if w.seen[v] {

family.go

@@ -103,8 +103,9 @@ func checkFamilies(reads []pathRead) error {
 }
 
 // replayPairs replays every two reads of one draw group that can render together into
-// one draws, the earlier read first. A pin conflicts with one earlier pin, never with a
-// combination, so pairs find every conflict a full replay would.
+// one draws, the earlier read first. Pairs find every conflict a full replay would: a
+// read pins a row's ancestors with it, and clash judges a row against one pinned table,
+// so a conflict is between two reads' pins and a third read adds nothing to it.
 func replayPairs(reads []pathRead) error {
 	for i, r := range reads {
 		if r.tr == nil {
@@ -216,8 +217,9 @@ func checkOwnFamily(t *template) error {
 	return nil
 }
 
-// alternatives reports whether two reads sit in different rows of one table: rows the walks pinned,
-// or rows of one whole draw.
+// alternatives reports whether two reads never render together: one read renders one row of a
+// table, so reads under different rows the walks pinned, or under different rows of one whole
+// draw, never meet.
 // docs/decisions.md#the-rows-of-a-table-are-alternatives
 func alternatives(a, b drawAt) bool {
 	return a.pins.differs(&b.pins) || a.wholePins.differs(&b.wholePins)

holdfence.go

@@ -18,7 +18,7 @@ func heldCheck(path string, n node) error {
 	readers := boundReaders(t.ops, t.bound)
 	for _, head := range heldHeads(t) {
 		if _, isPath := t.bound[head]; isPath && isRef(head) {
-			continue
+			continue // held for the render: drawCheck compares it across the render and its groups
 		}
 		if err := checkHeadHeld(t, head, readers); err != nil {
 			return fmt.Errorf("%s: %w", path, err)

todo.md

@@ -21,11 +21,6 @@ Self-sufficiency 5.6), every seat capped by Locality, and all nine naming
 
 This round, in order:
 
-- State at each draw fence, in one line at the code, the rule it enforces, where a
-  `docs/decisions.md` link stands in for it today: the clash drop in `drawWalk.walk`,
-  `alternatives`, `replayPairs`' claim that pairs find every conflict, and
-  `heldCheck`'s skip of a reference path. Seven seats could not say why the fences
-  are complete without the decision bodies.
 - Hold the draw fences to what render does: a test rendering seeded draws that fails
   where a read the render makes falls outside what `drawWalk` gathered. Today the two
   models agree by a comment on `renderEdges`, and a new render route drifts silently.