  We study the problem of converting a product of Greenberger-Horne-Zeilinger
(GHZ) states shared by subsets of several parties in an arbitrary way into GHZ
states shared by every party. Our result is that if SLOCC transformations are
allowed, then the best asymptotic rate is the minimum of bipartite log-ranks of
the initial state. This generalizes a result by Strassen on the asymptotic
subrank of the matrix multiplication tensor.
