How can I make Map property that holds Sets as values?

Table A ( a_id integer)
Table B (b_id integer)
Table C (c_id integer)
Table ABC (a_id integer (FK), b_id integer (FK), c_id integer (FK))

I'd like to make class A to have a Map property that maps B to set of C
for example
<class name="A" table="A">
<map name="B_to_C_map" table="ABC">
<key column="ReferenceElement_ID" />
<index-many-to-many column="b_id" class="B"/>
<!-- Something that defines Set of references of C instances -->
</map>
</class>

Should I consider <nested-composite-element> ???

Of cause I was incorrect in key column
<key column="a_id" />