Parallel by combination and link.

[Jin Li]

Hi,

I am trying to enumerate arguments by a combination of the first two
arguments but link the third. For example,

```
> parallel echo ::: 1 2 ::: a b :::+ x y z
1 a x
1 b y
2 a x
2 b y
```

The above example has generated the combination of the first two
arguments, i.e., {1, 2} and {a, b}. To link the third argument, it
ignored "z". I understand ":::+" will not wrap the third argument. I
am expecting the below output, i.e., to wrap the third argument when
linking the combinations of the first two.

```
1 a x
1 b y
2 a z
2 b x
```

Could you please help suggest how could I combine the combination of
the first two but link the third argument with wrapping? Thanks.

Best regards,
Jin