GADT problems

[Mitchell, Neil]

> Ah -- you used an *existential* there!  Yes, 
> existentially-bound type variables are rigid.  They stand for 
> themselves, as it were.

> How should this be clarified?

I'd leave it. I wanted a simple set of rules stating "_if_ you provide
the following type signatures your code _will_ compile", which is what
you currently provide, albeit I interpreted 'result' slightly
differently. If people want to learn where these type signatures can be
omitted (because a type is already rigid) people can follow the papers,
or learn by trial and error.

Thanks

Neil

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