False "duplicate or overlapping instances" message

[Keith Wansbrough]

> > You can now see that the two instance declarations overlap: their
> > right hand sides are in fact *identical*.  Remember that the
> > typechecker simply matches on the right-hand sides ("heads") of the
> > instance declarations.
> No they do not overlap, unless there is an a b satisfying NodeTypeConfigParms a b
> and ArcTypeConfigParms a b.  Which there aint.

Haskell makes the "open world" assumption... there may be new
instances added later, and the behaviour of a well-typed program
should not change if this occurs.

I'll defer further discussion of this to the experts (Mark Jones?).

--KW 8-)