Question: Overloading &+- to be a binary operator.

Hi, all. I'm not too confident in the results of Tolerances. I am very confident in the results of ScientificErrorAnalysis. Sadly, the Tolerances package has an awesome +- operator, while ScientificErrorAnalysis has the bulky Quantity() function. I'd like to define +- to be Quantity, something akin to this:

`&+-` := (a,b)->Quantity(a,b);

Sadly, 

1+`&+-`(2);

Error, invalid input: `&+-` uses a 2nd argument, b, which is missing

It seems to want me to do `&+-`(a,b).

What's strange is that I can use another operator just fine:

> `&x` := (a, b) -> Quantity(a, b) ;
(a, b) -> Quantity(a, b)
> 1 &x 2;
Quantity(1, 2)

I know it's possible, because the Tolerances package is able to overload &+-, so I'm wondering if anyone here knows how exactly this might be done.

Cheers,

Charles.