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Creates a new Index by copying a part of selected Index.
Given the following Index:
   1 (number of columns) "" (no column name)
   "dag allemaal"
   "hallo"
   "tot morgen"
   6 (number of elements)
   2
   1
   2
   3
   2
   3
The command Extract part... 1 2 gives you the new Index:
   1 (number of columns) "" (no column name)
   "dag allemaal"
   "hallo"
   "tot morgen"
   2 (number of elements)
   2
   1
Note that all classes stay intact and may have zero references like for example the "tot morgen" class.
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