Overlapping types

[Tom Pledger]

Tom Pledger writes:
 > [...]
 > 
 >     --Subtype should be transitive; I may have done something evil
 >     --here, because Classic Hugs (November 1999) with the `-98' flag
 >     --rejects this decl with the message `Undefined type variable "b"'
 >     instance (Subtype a b, Subtype b c) => Subtype a c where
 >         up   x = up (up x)
 >         down z = down z >>= down

In case anyone was wondering (or fuming about having explained it in a
previous thread;-) , b wasn't bound in the instance head but should
have been.