 change of directory when adding
    	directories to the stack, so only the stack is manipulated.
    
    Arguments:
      +N	Rotates the stack so that the Nth directory (counting
    	from the left of the list shown by `dirs', starting with
    	zero) is at the top.
    
      -N	Rotates the stack so that the Nth directory (counting
    	from the right of the list shown by `dirs', starting with
    	zero) is at the top.
    
      dir	Adds DIR to the directory stack at the top, making it the
    	new current working directory.
    
    The `dirs' builtin displays the directory stack.