Simplifier loop with arith_ss

[bacam]

The simplifier can loop on a simple expression containing dimindex when using arith_ss:

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       HOL-4 [Kananaskis 11 (stdknl, built Thu Oct 05 13:10:20 2017)]

       For introductory HOL help, type: help "hol";
       To exit type <Control>-D
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[Use-ing configuration file /home/bcampbe2/.hol-config.sml]
> load "wordsTheory";
val it = (): unit
> SIMP_CONV arith_ss [] ``n + dimindex (:'a) + dimindex (:'b) - 1``;
Exception- Interrupt raised

The problem appears to be that part of the simpset in src/num/arith/src/numSimps.sml uses a term comparison function that ignores types, and the two appearances of dimindex differ only in their types. Changing the comparison to Term.compare appears to work, but I don't know if there's some reason for using the particular ordering that's in numSimps.

[mn200]

Thanks for the bug report! I think you're right that we can simply replace the poor comparison in numSimps.