tcl::unsupported::inject
AMG: What is this new command? This [L1 ] appears to be the initial check-in, which says that it takes arguments [::tcl::unsupported::inject coroname command args] and makes the named command be the first thing the coroutine does the next time it's resumed.
DKF: It's supposed to be a mechanism for introducing a command into a coroutine that allows for things like debugging. It is, however, considered rather too difficult to use (or explain the exact semantics of) in its current incarnation.