private static void addCellsOf(Tensor a, Tensor b, Tensor.Builder builder, DoubleBinaryOperator combinator) {
        for (Iterator<Tensor.Cell> i = a.cellIterator(); i.hasNext(); ) {
            Map.Entry<TensorAddress, Double> aCell = i.next();
            var key = aCell.getKey();
            Double bVal = b.getAsDouble(key);
            if (bVal == null) {
                builder.cell(key, aCell.getValue());
            } else if (combinator != null) {
                builder.cell(key, combinator.applyAsDouble(aCell.getValue(), bVal));
            }
        }
    }
Returns the type resulting from applying Merge to the two given types
vespajlib/src/main/java/com/yahoo/tensor/functions/Merge.java