diff --git a/C/exo.c b/C/exo.c
index 9a7561279..a0f9585e1 100644
--- a/C/exo.c
+++ b/C/exo.c
@@ -381,6 +381,8 @@ Yap_ExoLookup(PredEntry *ap USES_REGS)
   }
   if (count) {
     yamop *code = LOOKUP(i, arity, j0, LOCAL_ibnds);
+    if (code == FAILCODE)
+      return code;
     if (i->is_udi) 
       return ((CEnterExoIndex)i->udi_first)(i PASS_REGS);
     else return code;
diff --git a/C/exo_udi.c b/C/exo_udi.c
index e9da05cab..aef754e36 100644
--- a/C/exo_udi.c
+++ b/C/exo_udi.c
@@ -90,7 +90,7 @@ compare(const BITS32 *ip, Int j USES_REGS) {
      BITS32 *sorted0, *sorted;
 
      /* be conservative */
-     sz = sizeof(BITS32)*(it->ntrys+it->nentries);
+     sz = sizeof(BITS32)*(2*it->ntrys+it->nentries);
      /* allocate space */
      if (!(it->udi_data = malloc(sz)))
        return;
diff --git a/H/clause.h b/H/clause.h
index eb4fd51d6..9d565a53a 100644
--- a/H/clause.h
+++ b/H/clause.h
@@ -176,7 +176,8 @@ typedef struct index_t {
   BITS32 *links;
   size_t size;
   yamop *code;
-  void *udi_data, *udi_first, *udi_next;
+  BITS32 *udi_data;
+  void *udi_first, *udi_next;
   UInt udi_arg;
 } Index_t;
 
diff --git a/docs/yap.tex b/docs/yap.tex
index b1ae24468..0fe01fe4d 100644
--- a/docs/yap.tex
+++ b/docs/yap.tex
@@ -9095,11 +9095,11 @@ This package assumes you use UDI exo-indexing, that is:
 @end example
 It is designed to optimise the following type of queries:
 @example
-?- max(X, diagnoses(X, 9, Y)).
+?- max(X, diagnoses(X, 9, Y), X).
 
-?- min(X, diagnoses(X, 9, 36211117)).
+?- min(X, diagnoses(X, 9, 36211117), X).
 
-?- X #< Y, min(X, diagnoses(9, X, 36211117)), diagnoses(Y, 9, _).
+?- X #< Y, min(X, diagnoses(X, 9, 36211117), X ), diagnoses(Y, 9, _).
 @end example
 The first argument gives the time, the second the patient, and the
 third the condition code. The first query should find the last time
diff --git a/library/exo_interval.yap b/library/exo_interval.yap
index b4098e4a0..a625386aa 100644
--- a/library/exo_interval.yap
+++ b/library/exo_interval.yap
@@ -89,6 +89,19 @@ X #=< Y :-
 	  X =< Y
         ).
 
+X #= Y :-
+	( var(X) -> insert_atts(X, i(Y-1,Y+1,_))
+                  ;
+	          ( var(Y) -> insert_atts(Y, i(X-1,X+1,_) ) ; 
+		      true
+		  )
+	      ;
+          var(Y) -> insert_atts(Y, i(X-1,X+Y,_))
+        ;
+	  X =:= Y
+        ).
+
+
 attribute_goals(X) -->
         { get_attr(X, exo_interval, Op) },
         ( { Op = max } -> [max(X)] ;