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