for a given mode, return the mode of a new instance of a oneway_monad. This returns either `plain` for a plain monad, or `repl` for a an instated effect.
0.095dev (2025-09-09 14:29:31 GIT hash 98644f8f651c2101a0730cfe31c5807993b7603b built by fridi@fzen)
This returns either `plain` for a plain monad, or `repl` for a an
instated effect.