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 (2025-08-15 12:02:22 GIT hash 301b5b75e77076d091b38f555473f9f0e31e5b5c built by fridi@fzen)
This returns either `plain` for a plain monad, or `repl` for a an
instated effect.