bestPos = b.Pos
			for _, v := range b.Values {
				if v.LackingPos() {
					continue
				}
				if v.Pos != src.NoXPos {
					// Assume values are still in roughly textual order;
					// TODO: could also seek minimum position?
					bestPos = v.Pos
					break
				}
			}
		}
		p.Pos = bestPos
		s.updateUnsetPredPos(p) // We do not expect long chains of these, thus recursion is okay.
	}
}