`toplevel.smi` includes declarations which you can use in toplevel of SML#.
Maybe, the definition of SML require this toplevel environment.
After including `_require "basis/main/toplevel.smi`, you can use some functions, ex. `^`, `foldl`.

In SML# separate compilation, all required files you write is linked.
So, if you use `_require "basis.smi"`, program size will increase.