Codiagonals in @program macro that are not returned

The @program macro allows for very easy wiring directed wiring diagram generation, but there are certain wiring diagrams it cannot express. For example, I'd like the following program to return `a` and merge inputs `b` and `c`
```
@program C (a::X,b::X,c::X) begin
[b, c]
return a
end
```

However, it just returns `a`. Any codiagonal that doesn't somehow make it to the final return expression gets ignored, it seems.
```
@program C (a::X,b::X,c::X) begin
return (a, [b, c])
end
```

This program almost gives us what we want, but it has b (which equals c) as an output that we don't want. A current workaround is to generate wiring diagrams like this and then trim off outputs that weren't actually desired from the ACSet.

(There's a dual issue when we want to create cups rather than caps, requiring us to create artificial inputs which then must be 'trimmed' at the end, too.)