Give a fence walk's pin set one copy, and file the drawn-row half of the pin model

family.go

@@ -225,7 +225,8 @@ func checkOwnFamily(t *template) error {
 	return nil
 }
 
-// alternatives reports whether two reads sit in different rows of one table.
+// alternatives reports whether two reads sit in different rows of one table. Not clash: a row drawn
+// whole is held without its ancestors, since the render draws it over the whole table.
 func alternatives(a, b drawAt) bool {
 	found := false
 	a.alt.each(func(t *table, r int) {

pins.go

@@ -163,10 +163,16 @@ func (p *pinSet) inside(t *table, rows []int) []int {
 	return rows
 }
 
+// clone is a copy of p that pins apart from it; a plain copy shares the spill map.
+func (p pinSet) clone() pinSet {
+	p.spill = maps.Clone(p.spill)
+	return p
+}
+
 // entered is p with row r of t: pinned where the render pins it, alone where the render draws it
 // to render whole. p is left as it was, since a fence walk branches.
 func (p pinSet) entered(t *table, r int, pins bool) pinSet {
-	p.spill = maps.Clone(p.spill)
+	p = p.clone()
 	switch _, in := p.pinned(t); {
 	case pins:
 		p.pin(t, r)

todo.md

@@ -43,7 +43,9 @@ This round, in order:
   decided by `indexOutside(body, '(')` in two of them and by `funcCall` in the rest.
 - Name `pathWalk`'s modes: render, probe, check and cover are chosen today by which
   callbacks are nil and whether its session is, and `walkChoice` returns `nil, nil` where
-  none is set. The mode may live in `pathWalk`; the session may not, since beside `pins` it
+  none is set. Have the check mode report the tables a read draws, so `tableRead.drawn`
+  and `entered`'s `pins` flag come from the walk rather than a second copy of
+  `walkTable`'s draw condition and `rowOf`'s ancestor rule. The mode may live in `pathWalk`; the session may not, since beside `pins` it
   moves the hold maps to the heap.
 - Register a transform once: `transforms` restates the `lowercase`, `uppercase` and
   `ascii` entries in `builtins`, and `valueProof.template` classifies by the first while