How to have strongly typed numerical values?

[ixid]

Using this:

struct Grams
{
     size_t amount;
}

@property Grams grams(size_t amount)
{
     return Grams(amount);
}

void main()
{
     auto weight = 5.grams;
     weight = weight + 10.grams;
}

How would you use it? I thought the point of this sort of strong 
typing was to be able to carry out arithmetic using your type 
that other units cannot accidentally be mixed with.