Report a held arm with no hold as an internal error, and name readHeld as the hold

doc.go

@@ -24,7 +24,7 @@ package fejkdata
 //     start one each, `FakeStruct` one per record, and `expandAnew` one per
 //     iteration of a `template.repeat`.
 //   - hold — keeping one draw of a name, so every route to it reads that value:
-//     `formatOps.held`, `hold`, `groupHold`, `readField`.
+//     `formatOps.held`, `hold`, `groupHold`, `readHeld`.
 //   - draw group — reference draws held apart inside one render:
 //     `template.drawGroup` as the data spells it, `templateLink.drawGroupKey` as a
 //     render reads it, `holdSet`, `renderScope`.

hold.go

@@ -41,6 +41,9 @@ func readField(s *session, t *template, held *hold, sc renderScope, a arm) readV
 		}
 		return readValue{text: render(s, t.head(a.key), sc)}
 	}
+	if held == nil {
+		panic(internalError("%q reads a name the expansion holds, with no hold to keep it in", a.spelling))
+	}
 	return readHeld(s, t, held, nil, sc, a)
 }