How to do Multi-Mapping based on one field?

Hi All,

I am now doing multi-mapping like this:

Source A:

<A>

<A1>

<A2>a</A2>

<A3>b</A3>

</A1>

<A1>

<A2>c</A2>

<A3>d</A3>

</A1>

<A1>

<A2>e</A2>

<A3>f</A3>

</A1>

</A>

Source B:

<B>

<B1>

<B2>e</B2>

<B3>x</B3>

</B1>

<B1>

<B2>a</B2>

<B3>y</B3>

</B1>

</B>

Target C's structure is the same as A.

What I want to do is: If the value of A2 equals the one value of B2s, mapping B2,B3 to C2,C3; else maping A2,A3 to C2,C3, like this:

<C>

<C1>

<C2>a</C2>

<C3>y</C3>

</C1>

<C1>

<C2>c</C2>

<C3>d</C3>

</C1>

<C1>

<C2>e</C2>

<C3>x</C3>

</C1>

</C>

What's more, all the B2 value set belongs to A2 value set.

Is it possible? Any comment or suggestion is grateful!

Regards,

Nick