thenIO removal

GHC.Base has a function, thenIO, that isn't used anywhere in the libraries or compiler, and isn't exported anywhere "public". But for some reason, it's listed in compiler/prelude/PrelNames.lhs, which causes a validation failure if I remove it. Is there a reason that a completely unused function is wired in? Is it a historical artifact, or an optimization that was never completed, or something else? Should I wipe it out of PrelNames, or do we want to use it for something? David