open schema

Ive posted aquestion similar to this one some time ago.
However, I would like to try and refine my question and ask it more acuratly :

Is there any possibility to define (in an xml schema) a type, containing only simple types, of which some are required, and all the others unknown.
In other words: define a type in which i know what elements are supposed to be present, but do not know which "might" be present.

eg:
the following type must contain 'a', 'b', and 'c', but might also contain some other elements. It is also important to state that the order af appearance is of no siginificance.

    <letters>
        <a/>
        <hollat/>
        <c/>
        <club/>
        <b/>
        <smit/>
        <pliq/>
    </letters>

I am searching for a solution in which all elements are of same namespaces.


Thanks for reading
    Hope to hear from any one of you
        Oded