static Tensor evaluate(Tensor a, Tensor b, TensorType mergedType, DoubleBinaryOperator combinator) {
        // Choose merge algorithm
        if (hasSingleIndexedDimension(a) && hasSingleIndexedDimension(b) && a.type().dimensions().get(0).name().equals(b.type().dimensions().get(0).name()))
            return indexedVectorMerge((IndexedTensor)a, (IndexedTensor)b, mergedType, combinator);
        else
            return generalMerge(a, b, mergedType, combinator);
    }
Returns the type resulting from applying Merge to the two given types
vespajlib/src/main/java/com/yahoo/tensor/functions/Merge.java