add stack class (LIFO)

@stack/length.m

+   function length = length(self)
+      length = 0;
+      node = self.stack_pointer;
+      while ~isempty(node)
+        length = length + 1;
+        node = next(node);
+      end
+    end
+

@stack/pop.m

+function element = pop(self)
+%increase the stack pointer and return 'top' node data
+% usage :
+%   element = pop(stack)
+
+% $Id$
+
+if isempty(self.stack_pointer)
+  element = {};
+else
+  element = data(self.stack_pointer);
+  self.stack_pointer = next(self.stack_pointer);
+end
+

@stack/push.m

+  function self = push(self, element)
+      self.stack_pointer = node(element, self.stack_pointer);
+    end
+

@stack/stack.m

+function s = stack
+%STACK constructor for stack object
+% stack is a data structure based on the principle of 
+% Last In First Out (LIFO)
+%
+% usage:
+%   q  = stack    - Default constructor, empty stack table
+%
+% example:
+%   q = stack 
+%   q = push(q, [1:2])
+%   q = push(q, ones(1,5))
+%   element = pop(q)
+%   q = push(q, zeros(1,3))
+%   etc ...
+%
+% $Id$
+
+% copy constructor
+% ----------------
+if nargin == 0  
+  self.stack_pointer = {};
+else
+  error('stack:stack', 'Invalid arguments.');
+end
+
+% class constructor
+% -----------------
+s = class(self,'stack');

@stack/top.m

+  function element = top(self)
+%return 'top' node
+% usage
+%   data = top(stack)
+
+% $Id: get.m 416 2009-02-13 07:55:41Z jgrelet $
+
+% return object prepared in data 
+% ------------------------------
+
+      if isempty(self.stack_pointer)
+        element = {};
+      else
+        element = data(self.stack_pointer);
+      end
+    end
+