Merge pull request #1707 from triska/reif_doc

DOC: add link to "Indexing dif/2" in DocLog format

src/lib/reif.pl

@@ -1,3 +1,16 @@
+/** Predicates from [*Indexing dif/2*](https://arxiv.org/abs/1607.01590).
+
+Example:
+
+```
+?- tfilter(=(a), [X,Y], Es).
+   X = a, Y = a, Es = "aa"
+;  X = a, Es = "a", dif:dif(a,Y)
+;  Y = a, Es = "a", dif:dif(a,X)
+;  Es = [], dif:dif(a,X), dif:dif(a,Y).
+```
+*/
+
 :- module(reif, [if_/3, (=)/3, (',')/3, (;)/3, cond_t/3, dif/3,
 		         memberd_t/3, tfilter/3, tmember/2, tmember_t/3,
 		         tpartition/4]).