F # Units, generality issues 2

Following from this question, I still seem to be fighting at the edges of what is possible, although I don't think I am doing anything especially at the edge of the bloodstream:

type Vector2d = { X: float<'u>; Y: float<'u> }

      

        
        
        
      

    

Gives error FS0039: The unit parameter u is not defined.

AND

type Vector2d = { X: float<_>; Y: float<_> }

      

        
        
        
      

    

Gives me error FS0191: Anonymous variable units are not allowed in this declaration.

In this case, functions can handle "common" units, but types cannot?