Hi Mira:

In the APPC language Bill is working on we can write:

Z{
  int f() via A to B (SummingAPPC, CountingAPPC);
}

I think this is important that we can give two or more APPCs
to the same traversal. And we might want to have collaboration
between the APPCs.

I guess we need to extend the syntax so that we can specify a mapping:

Z{
  int f() via A to B (SummingAPPC with ... , CountingAPPC with ... );
}

We need to typecheck that the two APPCs are compatible.
We need to agree on which APPC will return a value. The first one?

How do we want to handle this?

An alternative would be to define merge-composition of 
traversal-style APPCs
and only give one APPC to a traversal, always.

We would define 

MyAPPC = merge( components (SummingAPPC with ... , CountingAPPC ... )
                output from SummingAPPC
              )

Z{
  int f() via A to B MyAPPC with ... ;
}

-- Karl