Interesting cases would be
$[(a,b) \mid a \in A \wedge b \in B] = [(1,2),(1,2)]$ ?
$[x \mid c \in C \wedge x = c \mod 2] = [1,0,1]$ ?
The only kind of informal definition for multiset-builder notation I could find was a part in the preliminaries section of A Potpourri of Reason Maintenance Methods.
Are there other (more rigorous) studies on that topic?