put write_barrier in colSwap where there used to be one. Left cliff's write_barrier before the return also. Was failinig with just the write barrier before the return, and was failing before with just the original write_barrier. So maybe both are needed. see https://0xdata.atlassian.net/browse/HEX-607 for the NPE stack traces that happen before this push. Passes after this. (search for kbn: to find the write_barrier in Function.java