The tt(WARNING) macro takes one argument: text to display as a warning. The 
tt(yodl) program makes sure that before showing the text, the current file and 
line number are printed. Other than this, tt(WARNING) works just as
tt(TYPEOUT) (see section ref(typeout)). 

Note that an analogous macro tt(ERROR) exists, which prints a message and then
terminates the program (see section ref(error)).
