Raised in the discussion for WT-3739. To add WT_CURSOR::modify support to workgen, we need to support u, and figure out what modify calls should do. As in, how customizable do we need the modify call to be? For a straw man, one could assume length of modify vector and average size of the data for each entry could be parameters. Since we currently have a fixed size for keys and values, a simple approach would have all modify entries always swap N bytes for a different N bytes. Then the size of the final resulting value would be the same fixed size. If the size isn't fixed (and a lower size bound is not known), it's harder to come up with modify parameters that will always work.