How does Prism compute the Gehan-Breslow-Wilcoxon test for comparing survival curves?

The Gehan-Breslow-Wilcoxon test compares survival curves while giving more weight to earlier time points. It seems that this method was developed independently by the three people it is named after. We have not tried to track down the original references. Prism uses equation 10.2 from Machin's text referenced below. Here is a link to a scan of the two relevant pages from that book. 

D. Machin, YB Bheung, MKB Parmar (2006) Survival Analysis, A Practical Approach, Second edition, Wiley