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.
0.094dev (2025-07-10 15:41:47 GIT hash eafa42ae743de61b62f958948d7abb5fada7756f built by fridi@fzen)
This returns either `plain` for a plain monad, or `repl` for a an
instated effect.