private static Tensor generalMerge(Tensor a, Tensor b, TensorType mergedType, DoubleBinaryOperator combinator) {
        Tensor.Builder builder = Tensor.Builder.of(mergedType);
        addCellsOf(a, b, builder, combinator);
        addCellsOf(b, a, builder, null);
        return builder.build();
    }
Returns the type resulting from applying Merge to the two given types
vespajlib/src/main/java/com/yahoo/tensor/functions/Merge.java