Query syntax in ancestors/parent predicate

I tried to use `ancestors` predicate to define custom predicate like the following:

```
(org-ql-defpred project ()
"Match a project - task with a todo subtask or :project: tag."
:body (or (tags-local "project")
(and
(todo)
(descendants `(and
(or (todo) (done))
(level < ,org-inlinetask-min-level))))))

(org-ql-defpred subproject ()
"Match a subproject"
:body (and (project)
(ancestors '(project))))
```

However, such definition failed when searching for `(subproject)` query.

Instead, I had to use non-intuitive definition:

```
(org-ql-defpred subproject ()
"Match a subproject"
:body (and (project)
(ancestors 'project)))
```

It is either a bug or I missed something in the documentation.