html design questions

I am working on an x3d/vrml97 module very similar, as you can  
imagine, to the HTML module found in the smlnj-lib.

I was looking at the module and had some sml questions about it:

I've noticed a lot of this sort of thing:

     structure CellVAlign =
       struct
	type align = string ref
	val top = ref "TOP"
	val middle = ref "MIDDLE"
	val bottom = ref "BOTTOM"
	val baseline = ref "BASELINE"
	fun toString (ref s) = s
	val fromString = match [top, middle, bottom, baseline]
       end

Why are the vals assiged to sting ref instead of just a string ?

I'm also not clear on the point of using a structure.  Why not just  
use a type:

type CellVAlign = string

?

In any case a big thanks to whoever wrote it.  It's going to be very  
helpful !

Thanks

Brian


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