Short description of COLLECTION's benchmarks.
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bench1 directory: usage of feature `put' and `item' while reading the collection
   from the `lower' to the `upper' index. 
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bench2 directory: usage of feature `put' and `item' while reading the collection
   from the `upper' to the `lower' index.  (i.e. note that the
   traversal is reversed !).
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bench3 directory: usage of feature `add_first'.
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bench4 directory: usage of feature `add_last'.
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