+ , SysTools.Option input_fn
+ -- We hackily use Option instead of FileOption here, so that the file
+ -- name is not back-slashed on Windows. cpp is capable of
+ -- dealing with / in filenames, so it works fine. Furthermore
+ -- if we put in backslashes, cpp outputs #line directives
+ -- with *double* backslashes. And that in turn means that
+ -- our error messages get double backslashes in them.
+ -- In due course we should arrange that the lexer deals
+ -- with these \\ escapes properly.