@Override
    public TensorFunction<NAMETYPE> withArguments(List<TensorFunction<NAMETYPE>> arguments) {
        if ( arguments.size() != 2)
            throw new IllegalArgumentException("Join must have 2 arguments, got " + arguments.size());
        return new Join<>(arguments.get(0), arguments.get(1), combinator);
    }
Returns the type resulting from applying Join to the two given types
vespajlib/src/main/java/com/yahoo/tensor/functions/Join.java