% request IA.trf
ia = xff('*IA*.trf', 'Please select IA TRF file...');
if isempty(ia) || ...
   ~isxff(ia, 'trf')
    error('No IA TRF file selected.');
end

% request FA.trf
fa = xff('*FA*.trf', 'Please select FA TRF file...');
if isempty(fa) || ...
   ~isxff(fa, 'trf')
    error('No FA TRF file selected.');
end

% request ACPC.trf
acpc = xff('*ACPC*.trf', 'Please select ACPC file... (leave blank for ISO-voxel space)');

% request TAL.trf (if ACPC is not empty)
if ~isempty(acpc)
    tal = xff('*.TAL', 'Please select TAL file...');
end

% compile afs cell array
if ~isempty(acpc)
    if ~isempty(tal)
        acpc = {acpc, tal};
    else
        acpc = {acpc};
    end
else
    acpc = {};
end
afs = {ia, fa, acpc{:}};