@Override
    public List<TensorFunction<NAMETYPE>> arguments() {
        return List.of(argumentA, argumentB);
    }
An optimization for tensor expressions where a join immediately follows a
reduce. Evaluating this as one operation is significantly more efficient
than evaluating each separately.

This implementation optimizes the case where the reduce is done on the same
dimensions as the join. A particularly efficient evaluation is done if there
is one common dimension that is joined and reduced on, which is a common
case as it covers vector and matrix like multiplications.

@author lesters
vespajlib/src/main/java/com/yahoo/tensor/functions/ReduceJoin.java