-8017 An illegal (invisible, control) character has been found on line
line-number, character char-number. It has been replaced by a blank in the
listing, but it is still in the source (input) file, and should be removed
before attempting to compile again.
Using a word-processing editor in document mode instead of text mode,
striking nonprinting keys while you are editing the file, or data
corruption can introduce illegal characters into files.
