Sierksma's conjecture on the minimum number of Tverberg partitions

Let $m(d,r)$ denote the minimum number of unordered Tverberg partitions of a set of $(d+1)(r-1)+1$ points in $\mathbb{R}^d$, where a Tverberg partition of order $r$ is a partition into $r$ sets whose convex hulls have a common point. Sierksma's conjecture. The minimum number is

$$m(d,r)=((r-1)!)^d.$$

This is a long-standing conjecture concerning the extremal number of Tverberg partitions.