Re: Ann: SWI-Prolog 7.1.0

>This is all fine, until '[]' may appear as a token in the list.  In that

>case, flatten/2 just removes it.  Due to the ambiguity of the empty list,
>creating a multi list and call flatten/2 is considered bad programming.
>Without this ambiguity, using multi lists is safe and can be a handy
>solution, in particular for the not-so-experienced Prolog programmer.

Also for experienced programmers.  I had to resort to ugly tagging 
of data in expressions to distinguish [] as a list from atoms, not
to mention strings.