RELAX question re: required value

I have a schema that allows a bunch of different elements to be 
interleaved in any order; one of them is <collection>;  I want to 
require that a collection with value "all" be present, as well as 
allowing any number of other collections, in any order, using RELAX -- 
is that possible?  I can't seem to work out how, if it is.  For now I'm 
just using

element collection { text }+ &

Thanks

-Mike