Index: Extract part...

Creates a new Index by copying a part of selected Index.

## Example

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.