Concept generics

[icedquinn]

How do these work? I don't see them mentioned in the documentation. I did try 
something like this:
    
    
    type
       Vector3*[T] = concept V
          V.x is var T
          V.y is var T
    
    
    Run

But later attempts to use `Vector3[int]` failed saying I had supplied the 
`typedesc[int]` instead.