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7-bit vs. 8-bit: You were confusing tag encoding and 
INTEGER value encoding.

no way to encode a string with a length: suppose you want 
a 32 bit length, what's wrong with
struct.pack("l",len(s))+s

15-bit representation: I believe the add and sub 
implementations make use of the guarantee that a short 
won't overflow if the input fits into 15 bits.

Tim: BER = Basic Encoding Rules (as in the subtitle of 
X.690)

Even after all this discussion, I still cannot see why the 
existing libraries (including those offered for free by 
third parties) are not sufficient. It appears that Paul 
wants, among other things, that marshal becomes 
documented; it also appears that this won't hapen. 

What the other things are that Paul wants, I cannot tell, 
so I recommend to close this report with "won't fix". 

Paul, if you have a specific change that you want to be 
made, or a specific problem that you want to point out, 
please submit a new report. This issue "provide a 
documented serialization func" really ought to be closed 
as "Fixed"; xmlrpclib is already part of standard library 
and fits the original problem description:
- it is a library for serializing Python basic objects
- it is documented in the sense that the protocol is 
specified precisely enough to write interoperating 
implementations in other languages.