    Add directories to stack.
    
    Adds a directory to the top of the directory stack, or rotates
    the stack, making the new top of the stack the current working
    directory.  With no arguments, exchanges the top two directories.
    
    Options:
      -n	Suppresses the normal 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.
    
    Exit Status:
    Returns success unless an invalid argument is supplied or the directory
    change fails.
