new file mode 100644
--- /dev/null
+++ b/src/ov-null-mat.cc
@@ -0,0 +1,111 @@
+/*
+
+Copyright (C) 2008 Jaroslav Hajek
+
+This file is part of Octave.
+
+Octave 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 3 of the License, or (at your
+option) any later version.
+
+Octave 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 Octave; see the file COPYING.  If not, see
+<http://www.gnu.org/licenses/>.
+
+*/
+
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
+#include "ov-null-mat.h"
+#include "ops.h"
+#include "defun.h"
+
+DEFINE_OV_TYPEID_FUNCTIONS_AND_DATA (octave_null_matrix, "null_matrix", "double");
+
+const octave_value octave_null_matrix::instance (new octave_null_matrix ());
+
+static octave_base_value *
+default_null_matrix_numeric_conversion_function (const octave_base_value& a)
+{
+  CAST_CONV_ARG (const octave_null_matrix&);
+
+  return a.empty_clone ();
+}
+
+octave_base_value::type_conv_fcn
+octave_null_matrix::numeric_conversion_function (void) const
+{
+  return default_null_matrix_numeric_conversion_function;
+}
+
+DEFINE_OV_TYPEID_FUNCTIONS_AND_DATA (octave_null_str, "null_string", "char");
+
+const octave_value octave_null_str::instance (new octave_null_str ());
+
+static octave_base_value *
+default_null_str_numeric_conversion_function (const octave_base_value& a)
+{
+  CAST_CONV_ARG (const octave_null_str&);
+
+  return a.empty_clone ();
+}
+
+octave_base_value::type_conv_fcn
+octave_null_str::numeric_conversion_function (void) const
+{
+  return default_null_str_numeric_conversion_function;
+}
+
+DEFINE_OV_TYPEID_FUNCTIONS_AND_DATA (octave_null_sq_str, "null_sq_string", "char");
+
+const octave_value octave_null_sq_str::instance (new octave_null_sq_str ());
+
+static octave_base_value *
+default_null_sq_str_numeric_conversion_function (const octave_base_value& a)
+{
+  CAST_CONV_ARG (const octave_null_sq_str&);
+
+  return a.empty_clone ();
+}
+
+octave_base_value::type_conv_fcn
+octave_null_sq_str::numeric_conversion_function (void) const
+{
+  return default_null_sq_str_numeric_conversion_function;
+}
+
+DEFUN (isnull, args, ,
+  "-*- texinfo -*-\n\
+@deftypefn {Built-in Function} {} isnull (@var{x})\n\
+Return 1 if @var{x} is a special null matrix, string or single quoted string.\n\
+Indexed assignment with such a value as right-hand side should delete array elements.\n\
+This function should be used when overloading indexed assignment for user-defined \n\
+classes instead of @code{isempty}, to distinguish the cases:\n\
+@table @asis\n\
+@item @code{A(I) = []}\n\
+This should delete elements if @code{I} is nonempty.\n\
+@item @code{X = []; A(I) = X}\n\
+This should give an error if @code{I} is nonempty.\n\
+@end table\n\
+@end deftypefn")
+{
+  octave_value retval;
+
+  int nargin = args.length ();
+
+  if (nargin == 1 && args(0).is_defined ())
+    retval = args(0).is_null_value ();
+  else
+    print_usage ();
+
+  return retval;
+}
+