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 (2026-02-20 12:42:29 GIT hash dbcc3e4b3d2a59f66230b482cff8f0b018178d77 built by fridi@fzen)
This returns either `plain` for a plain monad, or `repl` for a an
instated effect.