Gets the table column index occupied by the child at a particular 1-D child index.
see get_index_at,
get_row_at_index
| this |
a pointer to the Table implementor on which to operate. |
| index |
the specified child index, zero-indexed. |
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a int indicating the first column spanned by the child of a table, at the specified 1-D (zero-offset)
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