range of algebra operators

Hello,

I'm a bit puzzled what exactly the result of the algebra operators defined in 
section 12.4 is. Is it a set of solution mappings or a multiset?
I guess it is a multiset, as suggested by the equations with the cardinalities 
in the definitions. Additionally, it makes more sense to me (esp. w.r.t. 
nesting of algebra operators). However, the definitions use braces which 
suggest sets, because braces are usually used to denote sets.

Thanks,
Olaf