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+ released to Laurent T.
+ Book and sheet objects can now be pickled and unpickled. Instead of
reading a large spreadsheet multiple times, consider pickling it once
and loading the saved pickle; can be much faster. Thanks to Laurent
Thioudellet for the enhancement request.
+ Using the mmap module can be turned off. But you would only do that
for benchmarking purposes.
+ Handling NUMBER records has been made faster