@Override
    public Tensor evaluate(EvaluationContext<NAMETYPE> context) {
        Tensor a = argumentA.evaluate(context);
        Tensor b = argumentB.evaluate(context);
        TensorType joinedType = outputType(a.type(), b.type());
        return evaluate(a, b, joinedType, combinator);
    }
Returns the type resulting from applying Join to the two given types
vespajlib/src/main/java/com/yahoo/tensor/functions/Join.java