Description
Construct an iterator for the given table. Each step in the iteration
returns a subtable for which the values of the given columns are
equal. By default the table will be sorted before the iteration is
started to achieve that all equal values are grouped together.
The optional order argument can be given to define how the table
has to be sorted. 'a' is ascending, 'd' is descending, other is don't care
(which comes to ascending). Instead of a single letter any word can be
used starting with a or d (e.g. asc).
The optional sort argument can be given to define if the table
has to be sorted. F means no sorting. The default T means sorting.
The default sort algorithm used is heapsort.
By giving the sort argument a string value, it can also be used to
specify another sort algorithm. 'n' is no sort, 'q' is quicksort,
'i' is insertion sort, other is heapsort. A full word can also be used.
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