Due to a numbering snafu in the 5.0 branch, the 5.2 branch was forked from
5.0.x instead of being cut from the tip of 5.1 at the time. To avoid confusion,
5.1 was renumbered to 5.3.  So 5.1.x and 5.3.x are interchangable and refer to
the same branch of code.