We can think of several things, e.g. it is not a generic object, meaning that other persons (users of the MOO) can't make a child of it (have a copy for themselves.
Another problem is security, as you can see anybody will be able to write something onto your ``holder'' or worse, erase it. This is something else you will have to learn if you feel getting into serious MOO programming.
See the generic object
tutorial (section 8.10 on page 
)
for learning how to make a generic holder and secure verbs.
Now you may have a functional holder running, but it is very plausible
that you didn't understand in detail what you did. Look at your code
and try to understand every line of your code. To do this you
can dump it by typing: ``@dump holder'' and the print it
on paper. You also should fix a few little things, e.g. change
the messages printed to the user, fix missing whitespaces from the
code you copied, etc.