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From: dewar@merv.cs.nyu.edu (Robert Dewar)
Subject: Re: Beware: Rep spec on an enumeration type causes code explosion
Date: 1997/12/12
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Joe said

<<I have to disagree somewhat.  We were using enumeration types as message
type IDs all over the place, not just at the edges.  For safety-critical
messages, the enumerations are not permitted to be numerically contiguous,
being required to differ in at least four bits from all other message type
integer codes.  This 4-bit requirement is intended to greatly reduce the
probability of message spoofing.  (I know this isn't by itself
sufficient.  This isn't the entire defense, only the part relevant to the
current issue.)
>>

As I said in previous messages, I consider this usage perfectly reasonable.
However, it is probably a good idea to consider that such non-contiguous
enumeration types should not be used in loops, array indexes, or succ and
pred.

Indeed a reasonable compromise on the language design issue (Tucker says
he would remove them completely) would have been to say that these 
operations (array indexing, looping, succ and pred) are not available
for enumeration types with enumeration representation clauses. You can
construct these effects if you like using Pos and Val, but then these
operations (with their attendant possible overhead) are explicit.