> For the complete documentation index, see [llms.txt](https://php-fhir-tools.ardenexal.net/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://php-fhir-tools.ardenexal.net/fhirpath/functions/navigation.md).

# Tree Navigation & Utility

## Tree navigation

| Function             | Description                                                                                                     | Example                      |
| -------------------- | --------------------------------------------------------------------------------------------------------------- | ---------------------------- |
| `children()`         | Returns all direct child nodes of each input item.                                                              | `Patient.children()`         |
| `descendants()`      | Returns all descendant nodes recursively (shorthand for `repeat(children())`).                                  | `Patient.descendants()`      |
| `repeat(projection)` | Applies the projection repeatedly, unioning results until no new items appear (transitive closure). Cycle-safe. | `Questionnaire.repeat(item)` |

## Utility

| Function                             | Description                                                                               | Example                                      |
| ------------------------------------ | ----------------------------------------------------------------------------------------- | -------------------------------------------- |
| `trace(name [, projection])`         | Logs diagnostic output and returns the input unchanged.                                   | `Patient.name.trace('names').given`          |
| `aggregate(aggregator [, init])`     | Reduces a collection to a single value; exposes `$this`, `$index`, and `$total`.          | `value.aggregate($this + $total, 0)`         |
| `iif(condition, ifTrue [, ifFalse])` | Conditional expression; returns `ifTrue` when the condition is true, otherwise `ifFalse`. | `iif(active, 'yes', 'no')`                   |
| `sort([key, ...])`                   | Sorts by natural order, or by one or more projection keys (use unary `-` for descending). | `name.given.sort()` / `item.sort(-priority)` |

{% hint style="info" %}
`trace()` writes to a PSR-3 logger when one is configured via `FHIRPathEvaluator::setLogger()`, otherwise it falls back to PHP's `error_log()`.
{% endhint %}


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