[Vala] Help with generating headers. Forward declaration?

[Sam Danielson <samdanielson gmail com>]

I have a convenience library written in Vala so I need to generate
headers. There are a few circular dependencies and adding the -H
switch to valac causes something like this.

libsqlpilot/flight.vala:8.10-8.17: error: The type name `Aircraft'
could not be found
                public Aircraft? aircraft {

The convenience library compiles without -H but then it isn't of much
use for the rest of the project.

Perhaps I need to add a forward declaration? How do I handle this in
Vala?

-Sam