This function determines how anything in `backquotes` is expanded on input.
FAILURE CONDITIONS
Never fails.
EXAMPLE
# quotexpander "1 + 1";;
val it : string = "parse_term \"1 + 1\""
# quotexpander ":num";;
val it : string = "parse_type \"num\""
COMMENTS
Not intended for general use, but automatically invoked when anything is typed
in backquotes `like this`. May be of some interest for users wishing to
change the behavior of the quotation parser.