diff --git a/C/save.c b/C/save.c
index ea8669eb0..cf143008f 100644
--- a/C/save.c
+++ b/C/save.c
@@ -2104,6 +2104,7 @@ RestoreClause(Clause *Cl)
     case _p_slr_vv:
     case _p_arg_vv:
     case _p_func2s_vv:
+    case _p_func2f_xx:
       pc->u.xxx.x  = XAdjust(pc->u.xxx.x);
       pc->u.xxx.x1 = XAdjust(pc->u.xxx.x1);
       pc->u.xxx.x2 = XAdjust(pc->u.xxx.x2);
@@ -2140,6 +2141,13 @@ RestoreClause(Clause *Cl)
       pc->u.xcx.xi = XAdjust(pc->u.xcx.xi);
       pc = NEXTOP(pc,xcx);
       break;
+      /* instructions type xyx */
+    case _p_func2f_xy:
+      pc->u.xyx.x = XAdjust(pc->u.xyx.x);
+      pc->u.xyx.x1 = XAdjust(pc->u.xyx.x1);
+      pc->u.xyx.y2  = YAdjust(pc->u.xyx.y2);
+      pc = NEXTOP(pc,xyx);
+      break;
       /* instructions type yxx */
     case _p_plus_y_vv:
     case _p_minus_y_vv:
@@ -2151,11 +2159,19 @@ RestoreClause(Clause *Cl)
     case _p_slr_y_vv:
     case _p_arg_y_vv:
     case _p_func2s_y_vv:
+    case _p_func2f_yx:
       pc->u.yxx.y  = YAdjust(pc->u.yxx.y);
       pc->u.yxx.x1 = XAdjust(pc->u.yxx.x1);
       pc->u.yxx.x2 = XAdjust(pc->u.yxx.x2);
       pc = NEXTOP(pc,yxx);
       break;
+      /* instructions type yyx */
+    case _p_func2f_yy:
+      pc->u.yyx.y1 = YAdjust(pc->u.yyx.y1);
+      pc->u.yyx.y2 = YAdjust(pc->u.yyx.y2);
+      pc->u.yyx.x = XAdjust(pc->u.yyx.x);
+      pc = NEXTOP(pc,yyx);
+      break;
       /* instructions type yxc */
     case _p_plus_y_vc:
     case _p_minus_y_cv:
diff --git a/changes4.3.html b/changes4.3.html
index 3908cd6ef..51cda48b8 100644
--- a/changes4.3.html
+++ b/changes4.3.html
@@ -6,6 +6,8 @@
 
 <H2 ALIGN=CENTER>Yap-4.3.19:</H2>
 <UL>
+      <LI> FIXED: ord_union/2 should not use use merge/3 but instead
+      ord_union/3 (report from Nicos Angelopoulos).
       <LI> FIXED: statistics/0 should report to user_error (report from Nicos
       Angelopoulos).
       <LI> FIXED: database could copy compiled floats,longs, and bigs
diff --git a/docs/yap.tex b/docs/yap.tex
index 9b9bd9d6a..4e42c3b44 100644
--- a/docs/yap.tex
+++ b/docs/yap.tex
@@ -6537,15 +6537,19 @@ True when @var{Numbers} is a list of integers, and @var{Total} is their sum.
 @cindex ordered set
 
 The following ordered set manipulation routines are available once
-included with the @code{use_module(library(ordsets))} command.
+included with the @code{use_module(library(ordsets))} command.  An
+ordered set is represented by a list having unique and ordered
+elements. Output arguments are guaranteed to be ordered sets, if the
+relevant inputs are. This is a slightly patched version of Richard
+O'Keefe's original library.
 
 @table @code
 @item list_to_ord_set(+@var{List}, ?@var{Set})
 @findex list_to_ord_set/2
 @syindex list_to_ord_set/2
 @cnindex list_to_ord_set/2
-Holds when @var{Set} is the ordered representation of the set represented
-by the unordered representation @var{List}.
+Holds when @var{Set} is the ordered representation of the set
+represented by the unordered representation @var{List}.
 
 @item merge(+@var{List1}, +@var{List2}, -@var{Merged})
 @findex merge/3
@@ -6553,6 +6557,10 @@ by the unordered representation @var{List}.
 @cnindex merge/3
 Holds when @var{Merged} is the stable merge of the two given lists.
 
+Notice that @code{merge/3} will not remove duplicates, so merging
+ordered sets will not necessarily result in an ordered set. Use
+@code{ord_union/3} instead.
+
 @item ord_add_element(+@var{Set1}, +@var{Element}, ?@var{Set2})
 @findex ord_add_element/3
 @syindex ord_add_element/3
diff --git a/library/ordsets.yap b/library/ordsets.yap
index 73421b49e..9cdde7fb1 100644
--- a/library/ordsets.yap
+++ b/library/ordsets.yap
@@ -316,16 +316,16 @@ ord_member(>,El,[H|T]) :-
 ord_union([], []).
 ord_union([Set|Sets], Union) :-
     length([Set|Sets], NumberOfSets),
-    merge_all(NumberOfSets, [Set|Sets], Union, []).
+    ord_union_all(NumberOfSets, [Set|Sets], Union, []).
 
-merge_all(N,Sets0,Union,Sets) :-
+ord_union_all(N,Sets0,Union,Sets) :-
     (  N=:=1  -> Sets0=[Union|Sets]
     ;  N=:=2  -> Sets0=[Set1,Set2|Sets], 
-                 merge(Set1,Set2,Union)
+                 ord_union(Set1,Set2,Union)
     ;  A is N>>1,
        Z is N-A,
-       merge_all(A, Sets0, X, Sets1),
-       merge_all(Z, Sets1, Y, Sets),
-       merge(X, Y, Union)
+       ord_union_all(A, Sets0, X, Sets1),
+       ord_union_all(Z, Sets1, Y, Sets),
+       ord_union(X, Y, Union)
     ).