Documentation 
Section: In the "Interface" section ov the Vala tutorial, on http://live.gnome.org/Vala/Tutorial#head-550883e2f1aeebc10f4d160561df2907ff79255c
In the "Interfaces" section of the Vala tutorial, interface definitions are not fully explained, and may easily lead to misinterpretations.

The following interface example doesn't compile:
--------
public interface ITest {
    public abstract int data_1 { get; set; }
    public abstract void function_1();
}
--------
--------
public class Test1 : ITest {
    public int data_1 { get; set; }
    public void function_1() {
    }
}
--------
--------
Test1 t = new Test1();
t.function_1();

ITest i = t;
i.function_1();
--------

(The error is: "missing class prerequisite for interface")

Correct version:
It should be clarified that such a use of interfaces needs them to have a class as prerequisite (e.g. GLib.Object), and the code example should be modified accordingly.

Other information:
The bug/Not-A-Bug #547362 should help to clarify this issue.