It's feature weeks for mksh(1). You almost certainly want to follow the mksh-current changelog. Please test all the new features, as well as make sure that nothing breaks nor I introduced other regressions, such as making resource usage worse (disc, CPU, memory). Please be reminded that many more deep-reaching changes will follow in the next few hacking days.
Thanks to David Korn (the Korn in AT&T Korn Shell, ksh88/ksh93) for discussing features and helping with finding syntax and semantic of some of the more complicated language pecularities.