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 (2025-10-09 18:01:25 GIT hash 4826e516431c193e428991dd546a46472973d8d4 built by fridi@fzen)
This returns either `plain` for a plain monad, or `repl` for a an
instated effect.