[Zope] Mixing keys

[Harm_Kirchhoff at mail.digital.co.jp]

I am new to ZOPE, so maybe this question is too simple:

I would like to use a BTRee (OOBTree) to store information.
For one type of sub-branch, I would like to use a continuous series of 
integers as keys:

dbroot['SE_HMI']={ '2003' : { 'jrl' : { 0 : <a tuple of data>}} 
dbroot['SE_HMI']={ '2003' : { 'jrl' : { 1 : <a tuple of data>}}
dbroot['SE_HMI']={ '2003' : { 'jrl' : { 2 : <a tuple of data>}} 
... and so on ...

all other branches besides 'jrl' will have string keys.

I read in the ZODB,ZEO programming guide (www.zope.org/Wikis/ZODB/guide/)
that you should only use objects of a SINGLE type as keys.

I could write a generator, that would create strings, by counting from 0 
to 255,
however, that would destroy the sort order, because 'ABX' would sort 
before 'X', allthough it would represent a much larger number.
Since the frequency of expected accesses will increase with the count 
(because the later records will be accessed more frequently), I expect 
that performance would be lower as the keys would end up in an unsorted 
manner.

The same problem occurs with strings representing numbers, due to the sort 
order:
cmp('123','2') , cmp(123,2) , "/" , cmp('ABX','X')
(-1, 1, '/', -1)

Does anybody have any experience with this problem ?