Editing Prolog (section)

=== Modes ===
{| style="float: right;" class="wikitable" border="1"
|-
! Mode specifier
! Interpretation
|-
| <code>+</code>
| <code>nonvar</code> on entry
|-
| <code>-</code>
| <code>var</code> on entry
|-
| <code>?</code>
| Not specified
|}
The syntax of Prolog does not specify which arguments of a predicate are inputs and which are outputs.<ref name="craft">{{cite book |author=O'Keefe, Richard A. |title=The craft of Prolog |publisher=MIT Press |location=Cambridge, Massachusetts |year=1990 |isbn=978-0-262-15039-2}}</ref> However, this information is significant and it is recommended that it be included in the comments.<ref>{{cite arXiv |title=Coding guidelines for Prolog |date=2010 |eprint=0911.2899 |last1=Covington |first1=Michael |last2=Bagnara |first2=Roberto |last3=O'Keefe |first3=Richard |last4=Wielemaker |first4=Jan |last5=Price |first5=Simon |display-authors=2 |class=cs.PL}}</ref> Modes provide valuable information when reasoning about Prolog programs<ref name="cite doi|10.1007/BF01213601"/> and can also be used to accelerate execution.<ref>{{Cite book |doi=10.1007/BFb0014976 |last1=Roy |first1=P. |last2=Demoen |first2=B. |isbn=978-3-540-17611-4 |last3=Willems |first3=Y. D. |chapter=Improving the execution speed of compiled Prolog with modes, clause selection, and determinism |title=Tapsoft '87 |series=Lecture Notes in Computer Science |volume=250 |pages=[https://archive.org/details/tapsoft87proceed0000inte/page/111 111] |year=1987 |chapter-url=https://archive.org/details/tapsoft87proceed0000inte/page/111}}</ref>