Add checking mode examples for append

[jgarte]

This pull request adds a checking mode/direction example for append.

[triska]

My personal opinion: Illustrating list predicates is a good opportunity to show how conveniently strings can be written in Scryer Prolog. For example:

?- append("Scr", "yer", "Scryer").
   true.

[jgarte]

My personal opinion: Illustrating list predicates is a good opportunity to show how conveniently strings can be written in Scryer Prolog. For example:

?- append("Scr", "yer", "Scryer").
true.

This sounds good to me. Should I update the proposed PR to use your string example in both modes for append/3?

[triska]

Personally, I would favour such a change, yes. @aarroyoc I would also greatly appreciate your opinion on this. Thank you a lot!

[triska]

Regarding the wording of the documentation for append/3, please also consider the Prologue:

https://www.complang.tuwien.ac.at/ulrich/iso-prolog/prologue#append

Note how generally phrased the sufficient condition is: "... true if Zs is the concatenation of the lists Xs and Ys."

It doesn't say "concatenate the lists" (which would suggest only a single usage mode), but "true if ...".

[jgarte]

@triska I'll wait on @aarroyoc 's opinion here for the PR.

[aarroyoc]

I agree with @triska that append/3 is a good place to showcase Scryer Prolog strings but I would keep at least one example with the [] syntax to not confuse people into thinking it's a predicate exclusively made for strings.

[jgarte]

@aarroyoc @triska Ok, I pushed a new commit with the suggestions. What do you think?

[triska]

Looks excellent to me, thank you a lot! Please consider also the description of the predicate if you are interested! Ideally the description also conveys the generality.

[jgarte]

Looks excellent to me, thank you a lot! Please consider also the description of the predicate if you are interested! Ideally the description also conveys the generality.

@triska Pushed a commit to improve that. Thanks for the suggestion!

a17e529