When defining an abstract method such as:
public abstract void some_method ();

It is interpreted as a vfunc, which is great, but in the gir output the instance parameter is missing, we get:

 <field name="some_method">
  <callback name="keyboard_some_method" c:type="keyboard_some_method">
   <return-value transfer-ownership="none">
    <type name="none"/>
   </return-value>
  </callback>
 </field>

Where we should get:
 <field name="some_method">
  <callback c:type="keyboard_some_method" name="keyboard_some_method">
   <parameters> 
    <parameter name="self" transfer-ownership="none">
     <type c:type="CaribouKeyboard" name="Keyboard"/>
    </parameter>
   </parameters>
   <return-value transfer-ownership="none">
    <type name="none"/>
   </return-value>
  </callback>
 </field>

This makes it hard to override the method properly in pygi since we can't pass the instance variable to the method.