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Currently the library reference contains minimal information
on internal objects; here's the entire text of the relevant
section:

"""
2.3.10.10 Internal Objects

See the Python Reference Manual for this information. It
describes stack frame objects, traceback objects, and slice
objects. 
"""

Perhaps cell and dictproxy types should be added to the
language reference manual.  On the other hand, it isn't
clear to me that absolute completeness is desirable.  Any
language or application has some undocumented features; the
fact that they 
are not documented is a signal to the user that their use is
not supported and they may go away without warning in future
versions.