Why can't you create a out of order subtype?

[brett_gengler@yahoo.com]

I created a type whose order can not be rearranged and I want to create
a subtype of that type.  So,

type msg is ( msg_a, msg_b, msg_c, msg_d, msg_e);

subtype vowel_msg is msg (msg_a, msg_e); --wrong, but why?


Is it true that I can't define a a subtype that's not a ordered
subrange of a type?  My issue is that I want to enforce the subtype at
compile time instead of creating an array of booleans and doing a run
time check. (This is for an interface and I don't trust people to send
me the right types).

vowel_filter is array(msg) of Boolean := (msg_a => True, msg_e => True,
others => False);  --yuck.

Another option that I think is much, much worse then the boolean array
would be create an overloaded type...

type msg is ( msg_a, msg_b, msg_c, msg_d, msg_e);
type vowelmsg ( msg_a, msg_e);

But as you know, msg.msg_e = 4 and vowelmsg.msg_e = 1 so an unchecked
conversion is wrong on so many levels.

Am I screwed?  Should I just implement a boolean array and shut up
about it?