Identify a fence alternative by the table row rather than the cell, keep the outermost row through a reached table, and key reads and visits by it

README.md

@@ -962,12 +962,15 @@ renamed or retyped line is a major.
   format and options, at the cost of holding the rows twice, which is what a
   category over a register with two natural formats asks for; a TSV nothing names
   stays a load error, since that one is a file forgotten rather than shared.
-- **The cells of one column are alternatives.** Only one row renders, so two cells
-  selecting different rows of another table never meet, as two items of a choice
-  never do; the family fence replays the reads of each cell apart, together with
-  the reads outside any cell. A path is walked once without drawing before it is
-  walked for real, so a path that fails below its first level moves no seeded
-  stream.
+- **The rows of a table are alternatives.** Only one row renders, so a cell in one
+  row and a cell in another never meet, and each may select its own row of another
+  table; the cells of one row, and whatever they reach, do meet, and so does the
+  format beside them. The family fence replays the reads of each row apart,
+  together with the reads outside any row. A choice's items get no such treatment
+  yet: two items selecting different rows are still refused.
+- **A path is walked once without drawing before it is walked for real.** A path
+  that fails below its first level then moves no seeded stream, at the cost of one
+  draw-free walk per call, which allocates nothing.
 - **`List` advertises direct descents only.** `region.municipality.locality` is
   listed, and `region.locality` resolves too but is not: the set of every descent
   through a chain of five tables is every subsequence of it, and the direct chain is

draw.go

@@ -271,17 +271,23 @@ type drawWalk struct {
 }
 
 // drawAt is where a walk stands: the draw group it draws in, how the render's root reached it, and
-// the table cell it entered, if any: the cells of one column are alternatives, as a choice's items
-// are, so reads in two of them never meet.
+// the table row it entered, if any.
 type drawAt struct {
 	group string
 	route drawRoute
-	cell  node
+	alt   rowAlt
 }
 
-// alternatives reports whether two reads sit in different cells of one column.
+// rowAlt is one row of a table as an alternative: only one row renders, so reads in two rows of
+// one table never meet, while reads in one row, across its columns and whatever they reach, do.
+type rowAlt struct {
+	t   *table
+	row int
+}
+
+// alternatives reports whether two reads sit in different rows of one table.
 func alternatives(a, b drawAt) bool {
-	return a.cell != nil && b.cell != nil && a.cell != b.cell
+	return a.alt.t != nil && a.alt.t == b.alt.t && a.alt.row != b.alt.row
 }
 
 // drawRoute is how a render reaches a draw: as its author spells it, and the root edge's label.
@@ -297,11 +303,13 @@ type pathRead struct {
 type drawVisit struct {
 	n     node
 	group string
+	alt   rowAlt
 }
 
 type drawKey struct {
 	group string
 	path  string
+	alt   rowAlt
 }
 
 func newDrawWalk() *drawWalk {
@@ -311,7 +319,7 @@ func newDrawWalk() *drawWalk {
 // walk follows what rendering n renders. A repeat renders over draws of its own, so the walk stops
 // there.
 func (w *drawWalk) walk(n node, at drawAt) {
-	v := drawVisit{n, at.group}
+	v := drawVisit{n, at.group, at.alt}
 	if w.seen[v] {
 		return
 	}
@@ -324,16 +332,18 @@ func (w *drawWalk) walk(n node, at drawAt) {
 	}
 	_, isColumn := n.(*column)
 	for _, e := range renderEdges(n) {
-		if isColumn {
-			at.cell = e.to
+		// The outermost row is kept: a cell reached through another row's cell renders with it.
+		edgeAt := at
+		if cell, _ := e.to.(*template); isColumn && at.alt.t == nil {
+			edgeAt.alt = rowAlt{cell.cellOf, cell.cellRow}
 		}
-		w.edge(n, e, at)
+		w.edge(n, e, edgeAt)
 	}
 }
 
 func (w *drawWalk) edge(from node, e renderEdge, at drawAt) {
 	if a, reads := refRead(from, e.label); reads {
-		if k := (drawKey{at.group, a.path}); !w.read[k] {
+		if k := (drawKey{at.group, a.path, at.alt}); !w.read[k] {
 			w.read[k] = true
 			w.reads = append(w.reads, pathRead{at, a, tableReadOf(from.(*template).fields[a.key], a, e.to)})
 		}

family.go

@@ -243,28 +243,39 @@ func checkFamilies(reads []pathRead) error {
 	return replayAlternatives(reads)
 }
 
-// replayAlternatives replays the reads of each draw group into one draws per cell
-// alternative — the reads outside any cell, then the cell's own — so two cells of
-// one column are never replayed together.
+// replayAlternatives replays the reads of each draw group into one draws per row
+// alternative — the reads outside any row, then the row's own — so two rows of one
+// table are never replayed together.
 func replayAlternatives(reads []pathRead) error {
 	type alt struct {
 		group string
-		cell  node
+		row   rowAlt
 	}
 	var alts []alt
-	seen := map[alt]bool{}
+	outside, inside := map[string][]pathRead{}, map[alt][]pathRead{}
 	for _, r := range reads {
-		if a := (alt{r.at.group, r.at.cell}); r.tr != nil && !seen[a] {
-			seen[a] = true
-			alts = append(alts, a)
-		}
-	}
-	for _, a := range alts {
-		var d draws
-		for _, r := range reads {
-			if r.tr == nil || r.at.group != a.group || r.at.cell != nil && r.at.cell != a.cell {
+		if r.tr == nil {
 			continue
 		}
+		if r.at.alt.t == nil {
+			outside[r.at.group] = append(outside[r.at.group], r)
+			continue
+		}
+		a := alt{r.at.group, r.at.alt}
+		if inside[a] == nil {
+			alts = append(alts, a)
+		}
+		inside[a] = append(inside[a], r)
+	}
+	for group := range outside {
+		alts = append(alts, alt{group, rowAlt{}})
+	}
+	sort.Slice(alts, func(i, j int) bool {
+		return altOrder(alts[i].group, alts[i].row) < altOrder(alts[j].group, alts[j].row)
+	})
+	for _, a := range alts {
+		var d draws
+		for _, r := range append(append([]pathRead(nil), outside[a.group]...), inside[a]...) {
 			if err := r.tr.replay(&d); err != nil {
 				return fmt.Errorf("%s: %w; select the same rows in every path into the family, or draw them apart with a drawGroup", r.at.route.spelled(r.a.name), err)
 			}
@@ -273,6 +284,15 @@ func replayAlternatives(reads []pathRead) error {
 	return nil
 }
 
+// altOrder sorts alternatives by group, the reads outside any row first, then by table and row,
+// so which conflict is reported does not vary.
+func altOrder(group string, r rowAlt) string {
+	if r.t == nil {
+		return group + "\x00"
+	}
+	return fmt.Sprintf("%s\x01%s\x00%08d", group, r.t.category, r.row)
+}
+
 // replay pins the read's rows into d, where they agree with the rows pinned before.
 func (r *tableRead) replay(d *draws) error {
 	var err error

node.go

@@ -66,6 +66,7 @@ type template struct {
 	record       bool        // compiled at the top without a repeat, so its fields are record columns
 	table        *table      // the table whose format this is, whose columns are the fields
 	cellOf       *table      // the table whose cell this is
+	cellRow      int         // the row the cell sits in
 	drawGroup    string      // the draw group it draws in, as written; "" keeps its caller's
 	drawGroupKey string      // its draw group keyed by its category once linked: what a render reads its reference paths under
 }

table.go

@@ -296,7 +296,7 @@ func (t *table) checkCells() error {
 			t.tokens = map[int]*template{}
 		}
 		t.tokens[i] = n.(*template)
-		t.tokens[i].cellOf = t
+		t.tokens[i].cellOf, t.tokens[i].cellRow = t, row
 	}
 	return nil
 }