This will default to PROD (as the 0th enum value), which should probably be made explicit by `[default = PROD]`.  I'm kind of conflicted about that default though -- sum is pretty clearly the more useful of the two ops and makes more intuitive sense as a default, but making that the default would require people upgrading prototxts (assuming anyone actually uses `ELTWISE_PRODUCT` in their prototxts...) to not only change to `ELTWISE` but also add a `eltwise_param { stuff }` to it.  I dunno, your call.  (If you wanted to be super fancy, you could add some kind of layer deprecation/aliasing logic into LayerFactory to convert a layer name to a different one and fill in any LayerParameters appropriately, but that might be a bit much to ensure a smooth transition for this tiny use case...)

Also, could there be a `repeated float coeff = 2;` or the like to specify coefficients for the sum inputs (a la cuda-convnet)? Useful especially for taking the difference between two layers with `coeff: 1.0\ncoeff: -1.0` (and in the implementation I think if you use `axpy` with `alpha=1` it falls back to `sum`).