Get an iterator to the indexed child of a parent
| return | child | parent | index | description |
| true | valid | null | valid | First iterator of the root |
| true | valid | valid | valid | First iterator of the parent |
| false | invalid | null | valid | Root has no children |
| false | invalid | valid | valid | Parent has no children |
| false | invalid | invalid | Logic error | |
| false | invalid | invalid | Logic error |
| child |
An iterator to the indexed child of the parent or invalid if the parent has no children. This invalid iterator is indicated by the return value. |
| parent |
An iterator to the parent or null if the root |
|
True when first represents a valid iterator |