new file mode 100644
--- /dev/null
+++ b/lib/gl_array_oset.c
@@ -0,0 +1,280 @@
+/* Ordered set data type implemented by an array.
+   Copyright (C) 2006 Free Software Foundation, Inc.
+   Written by Bruno Haible <bruno@clisp.org>, 2006.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software Foundation,
+   Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
+
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
+
+/* Specification.  */
+#include "gl_array_oset.h"
+
+#include <stdlib.h>
+
+#include "xalloc.h"
+
+/* Checked size_t computations.  */
+#include "xsize.h"
+
+/* -------------------------- gl_oset_t Data Type -------------------------- */
+
+/* Concrete gl_oset_impl type, valid for this file only.  */
+struct gl_oset_impl
+{
+  struct gl_oset_impl_base base;
+  /* An array of ALLOCATED elements, of which the first COUNT are used.
+     0 <= COUNT <= ALLOCATED.  */
+  const void **elements;
+  size_t count;
+  size_t allocated;
+};
+
+static gl_oset_t
+gl_array_create_empty (gl_oset_implementation_t implementation,
+		       gl_setelement_compar_fn compar_fn)
+{
+  struct gl_oset_impl *set =
+    (struct gl_oset_impl *) xmalloc (sizeof (struct gl_oset_impl));
+
+  set->base.vtable = implementation;
+  set->base.compar_fn = compar_fn;
+  set->elements = NULL;
+  set->count = 0;
+  set->allocated = 0;
+
+  return set;
+}
+
+static size_t
+gl_array_size (gl_oset_t set)
+{
+  return set->count;
+}
+
+static size_t
+gl_array_indexof (gl_oset_t set, const void *elt)
+{
+  size_t count = set->count;
+
+  if (count > 0)
+    {
+      gl_setelement_compar_fn compar = set->base.compar_fn;
+      size_t low = 0;
+      size_t high = count;
+
+      /* At each loop iteration, low < high; for indices < low the values
+	 are smaller than ELT; for indices >= high the values are greater
+	 than ELT.  So, if the element occurs in the list, is at
+	 low <= position < high.  */
+      do
+	{
+	  size_t mid = low + (high - low) / 2; /* low <= mid < high */
+	  int cmp = (compar != NULL
+		     ? compar (set->elements[mid], elt)
+		     : (set->elements[mid] > elt ? 1 :
+		        set->elements[mid] < elt ? -1 : 0));
+
+	  if (cmp < 0)
+	    low = mid + 1;
+	  else if (cmp > 0)
+	    high = mid;
+	  else /* cmp == 0 */
+	    /* We have an element equal to ELT at index MID.  */
+	    return mid;
+	}
+      while (low < high);
+    }
+  return (size_t)(-1);
+}
+
+static bool
+gl_array_search (gl_oset_t set, const void *elt)
+{
+  return gl_array_indexof (set, elt) != (size_t)(-1);
+}
+
+/* Ensure that set->allocated > set->count.  */
+static void
+grow (gl_oset_t set)
+{
+  size_t new_allocated;
+  size_t memory_size;
+  const void **memory;
+
+  new_allocated = xtimes (set->allocated, 2);
+  new_allocated = xsum (new_allocated, 1);
+  memory_size = xtimes (new_allocated, sizeof (const void *));
+  if (size_overflow_p (memory_size))
+    /* Overflow, would lead to out of memory.  */
+    xalloc_die ();
+  memory = (const void **) xrealloc (set->elements, memory_size);
+  if (memory == NULL)
+    /* Out of memory.  */
+    xalloc_die ();
+  set->elements = memory;
+  set->allocated = new_allocated;
+}
+
+/* Add the given element ELT at the given position,
+   0 <= position <= gl_oset_size (set).  */
+static inline void
+gl_array_add_at (gl_oset_t set, size_t position, const void *elt)
+{
+  size_t count = set->count;
+  const void **elements;
+  size_t i;
+
+  if (count == set->allocated)
+    grow (set);
+  elements = set->elements;
+  for (i = count; i > position; i--)
+    elements[i] = elements[i - 1];
+  elements[position] = elt;
+  set->count = count + 1;
+}
+
+/* Remove the element at the given position,
+   0 <= position < gl_oset_size (set).  */
+static inline void
+gl_array_remove_at (gl_oset_t set, size_t position)
+{
+  size_t count = set->count;
+  const void **elements;
+  size_t i;
+
+  elements = set->elements;
+  for (i = position + 1; i < count; i++)
+    elements[i - 1] = elements[i];
+  set->count = count - 1;
+}
+
+static bool
+gl_array_add (gl_oset_t set, const void *elt)
+{
+  size_t count = set->count;
+  size_t low = 0;
+
+  if (count > 0)
+    {
+      gl_setelement_compar_fn compar = set->base.compar_fn;
+      size_t high = count;
+
+      /* At each loop iteration, low < high; for indices < low the values
+	 are smaller than ELT; for indices >= high the values are greater
+	 than ELT.  So, if the element occurs in the list, is at
+	 low <= position < high.  */
+      do
+	{
+	  size_t mid = low + (high - low) / 2; /* low <= mid < high */
+	  int cmp = (compar != NULL
+		     ? compar (set->elements[mid], elt)
+		     : (set->elements[mid] > elt ? 1 :
+			set->elements[mid] < elt ? -1 : 0));
+
+	  if (cmp < 0)
+	    low = mid + 1;
+	  else if (cmp > 0)
+	    high = mid;
+	  else /* cmp == 0 */
+	    return false;
+	}
+      while (low < high);
+    }
+  gl_array_add_at (set, low, elt);
+  return true;
+}
+
+static bool
+gl_array_remove (gl_oset_t set, const void *elt)
+{
+  size_t index = gl_array_indexof (set, elt);
+  if (index != (size_t)(-1))
+    {
+      gl_array_remove_at (set, index);
+      return true;
+    }
+  else
+    return false;
+}
+
+static void
+gl_array_free (gl_oset_t set)
+{
+  if (set->elements != NULL)
+    free (set->elements);
+  free (set);
+}
+
+/* --------------------- gl_oset_iterator_t Data Type --------------------- */
+
+static gl_oset_iterator_t
+gl_array_iterator (gl_oset_t set)
+{
+  gl_oset_iterator_t result;
+
+  result.vtable = set->base.vtable;
+  result.set = set;
+  result.count = set->count;
+  result.p = set->elements + 0;
+  result.q = set->elements + set->count;
+
+  return result;
+}
+
+static bool
+gl_array_iterator_next (gl_oset_iterator_t *iterator, const void **eltp)
+{
+  gl_oset_t set = iterator->set;
+  if (iterator->count != set->count)
+    {
+      if (iterator->count != set->count + 1)
+	/* Concurrent modifications were done on the set.  */
+	abort ();
+      /* The last returned element was removed.  */
+      iterator->count--;
+      iterator->p--;
+      iterator->q--;
+    }
+  if (iterator->p < iterator->q)
+    {
+      const void **p = (const void **) iterator->p;
+      *eltp = *p;
+      iterator->p = p + 1;
+      return true;
+    }
+  else
+    return false;
+}
+
+static void
+gl_array_iterator_free (gl_oset_iterator_t *iterator)
+{
+}
+
+
+const struct gl_oset_implementation gl_array_oset_implementation =
+  {
+    gl_array_create_empty,
+    gl_array_size,
+    gl_array_search,
+    gl_array_add,
+    gl_array_remove,
+    gl_array_free,
+    gl_array_iterator,
+    gl_array_iterator_next,
+    gl_array_iterator_free
+  };

new file mode 100644
--- /dev/null
+++ b/lib/gl_array_oset.h
@@ -0,0 +1,35 @@
+/* Ordered set data type implemented by an array.
+   Copyright (C) 2006 Free Software Foundation, Inc.
+   Written by Bruno Haible <bruno@clisp.org>, 2006.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program; if not, write to the Free Software Foundation,
+   Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.  */
+
+#ifndef _GL_ARRAY_OSET_H
+#define _GL_ARRAY_OSET_H
+
+#include "gl_oset.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+extern const struct gl_oset_implementation gl_array_oset_implementation;
+#define GL_ARRAY_OSET &gl_array_oset_implementation
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _GL_ARRAY_OSET_H */

new file mode 100644
--- /dev/null
+++ b/modules/array-oset
@@ -0,0 +1,26 @@
+Description:
+Ordered set data type implemented by an array.
+
+Files:
+lib/gl_array_oset.h
+lib/gl_array_oset.c
+
+Depends-on:
+oset
+xalloc
+xsize
+
+configure.ac:
+
+Makefile.am:
+lib_SOURCES += gl_array_oset.h gl_array_oset.c
+
+Include:
+"gl_array_oset.h"
+
+License:
+GPL
+
+Maintainer:
+Bruno Haible
+