8.1.8 - Custom form controls

In the description of the xforms:trigger element it is stated, "This form 
control may also be used to construct other custom form controls.".

However there is no cross reference to any other part of the specification 
nor does the term "custom form control" appear to be present anywhere else in 
the document.

It seems that some additional explanation of what mechanisms are envisaged 
and what is their scope should be provided.

Andrew Watt