filename completions with prefixes

[Wataru Kagawa <wkagawa@xxxxxxxxxxxxxxxxxxxx>]

I am trying to write a completion function for a command called pdbset 
in which *.pdb files are completed with the prefixes 'XYZIN' and 
'XYZOUT'.  Below is an example:

pdbset XYZIN input.pdb XYZOUT output.pdb

So far I have,

local expl
_description files expl 'pdb files'
_path_files "$expl[@]" -g '*.pdb' || _path_files "$expl[@]" -/ -g 
'*.pdb'

I am having trouble inserting the prefixes.
Help is greatly appreciated.

Wataru Kagawa