Give a pick key one type, pickKey, and key every read's levels from the head its steps start at to its leaf

render.go

@@ -146,7 +146,7 @@ func expand(s *drawstate.State, t *template, sc renderScope) string {
 // a level that a read of the name being rendered addresses; every other read draws afresh, so
 // {word} {word} draws twice and {p.a} {p.b} reads two draws of p.
 // A fresh read clears sc.pick. Kept, a read nested in the fresh one would key the pick's memo by
-// sc.pickKey, the path from the name to what the pick renders, and share a value with a different
+// sc.pickAt, where in the pick it renders, and share a value with a different
 // node at that path.
 func readField(s *drawstate.State, t *template, sc renderScope, a arm) readValue {
 	switch {