Triple hat call in child document doesn't recognise function in parent document

[jwoLondon]

Suppose we have a function defined in a document:

parent.md

```elm{l=hidden}
myFunction : String
myFunction =
    "Hello from parent"
```

And if a child document that follows it makes a call to the parent via a triple hat expression:

child.md

---
follows: parent
---

^^^elm {r=myFunction}^^^

This results in the error I cannot find a 'myFunction' variable

However if some other function is declared in the child document, the function in the parent is called correctly:

---
follows: parent
---

^^^elm {r=myFunction}^^^

```elm {l=hidden}
anotherFunction : Int
anotherFunction =
    7
```

It would appear that there must be at lest one declared function within a document before a triple hat expression can reference a function in parent (or grandparent etc.) document. This will prevent 'clean' documents from being created that have no code but reference functions from other documents in the hierarchy.