Nat. Methods 14, 757–758 (2017); published online 28 July 2017; corrected after print 28 July 2017
In the version of this article initially published, the expression (g1, g2) used to describe a sample subset in the Figure 1 legend was incorrect. The correct expression is (Ig(S1), Ig(S2)). The error has been corrected in the HTML and PDF versions of the article.