Those two symbols are merely to make the code easier to read as there are a lot of thunks and forcing. Previously it was Fay$$force and Fay$$thunk. Technically

Those two symbols are merely to make the code easier to read as there are a lot of thunks and forcing. Previously it was Fay$$force and Fay$$thunk. Technically these are global within the Fay output, but that is all wrapped up in a closure, so it won't interfere with anything outside. Additionally, _ is not a valid function name in Haskell, and $ (and any other non-letter symbols) are encoded as $123$ where 123 is the unicode point. So these two symbols are free to use, if that bothered you at all. enumFromTo is defined in the Prelude.