Nature 430, 51–53 (2004).

In this Letter, Rainer Kuschnig's surname was misspelled as ‘Kusching’ in the author list. In addition, the numbering of the reference list was incorrect. References 1 to 26 should be, respectively: 1, 10–16, 2, 3, 17–26 and 4–9. In ref. 23 (ref. 6), ‘Rettler’ should read ‘Retter’.