@Override
    public String toString(ToStringContext<NAMETYPE> context) {
        return "reduce_join(" + argumentA.toString(context) + ", " +
               argumentB.toString(context) + ", " +
               combinator + ", " +
               aggregator +
               Reduce.commaSeparatedNames(dimensions, context) + ")";
    }
Tests if there is exactly one reduce dimension and it is the innermost
dimension in both tensors.
vespajlib/src/main/java/com/yahoo/tensor/functions/ReduceJoin.java