In the process of factoring out the field filtering, the "extra"
and "fullname" args are just passed in as a blank string now.
Extra was functionality that allowed a field modifier to be defined
as "filtername(arg1,arg2):field", and fullname was the name of the
field including any provided field modifiers. From grepping through
the add-ons on AnkiWeb, neither appears to have been used.
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