relaxed instance rules spec (was: the MPTC Dilemma(pleasesolve))

[Claus Reinke]

> the translation into CHRs would be something like:
> 
>    class C a b | a -> b
>    instance ctxt => C t1 t2
> 
> -->
> 
>    C a b <=> infer_C a b, memo_C a b.        (two roles)
> 
>    infer_C a b <=> ctxt.                                 (instance inference)
> 
>    memo_C  a b1, memo_C a b2 => b1=b2. (class FD)
> 
> so there'd be one preparatory inference rule to split all constraints 
> occurring in the process into their two roles, one inference rule for 
> each instance declaration, and one propagation rule for each FD.
> the CHR succeeds if there are no non-memo constraints left.

to avoid misunderstandings: this is not yet complete. it is sufficient
for the example from the paper, because the FD-based improvements
there are triggered by the two goal constraints, but the translation also
needs to account for FD-related information taken from the instance
declarations.

claus