Get an iterator to the first child of a parent
| return | first | parent | description |
| true | valid | null | First iterator of the root |
| true | valid | valid | First iterator of the parent |
| false | invalid | null | Root has no children |
| false | invalid | valid | Parent has no children |
| false | invalid | invalid | Logic error |
| first |
An iterator to the first 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 |