function trl = mytrialfun(cfg);

% this function requires the following fields to be specified
% cfg.dataset
% cfg.trialdef.eventtype
% cfg.trialdef.eventvalue
% cfg.trialdef.prestim
% cfg.trialdef.poststim

hdr   = ft_read_header(cfg.dataset);
event = ft_read_event(cfg.dataset);
markerfile = [cfg.dataset(1:end-3) 'mrk'];
 
trl = [];

if exist(markerfile,'file')
    % read the marker file and make an event for each marker
    % this depends on the readmarkerfile function that I got from Tom Holroyd
    % I have not tested this myself extensively, since at the FCDC we
    % don't use the marker files
    mrk = readmarkerfileDS(markerfile);
    for i=1:mrk.number_markers
        for j=1:mrk.number_samples(i)
            % determine the location of the marker, expressed in samples
            trialnum = mrk.trial_times{i}(j,1);
            synctime = mrk.trial_times{i}(j,2);
            begsample = (trialnum-1)*hdr.nSamples + 1;    % of the trial, relative to the start of the datafile
            endsample = (trialnum  )*hdr.nSamples;        % of the trial, relative to the start of the datafile
            offset    = round(synctime*hdr.Fs);           % this is the offset (in samples) relative to time t=0 for this trial
            offset    = offset + hdr.nSamplesPre;         % and time t=0 corrsponds with the nSamplesPre'th sample
            % store this marker as an event
            event(end+1).type    = mrk.marker_names{i};
            event(end ).value    = 0 ;
            event(end ).sample   = begsample + offset;
            event(end ).duration = 0;
            event(end ).offset   = offset;
        end
    end
end

 
for i=1:length(event)
  if strcmp(event(i).type, cfg.trialdef.eventtype)
    % it is a trigger, see whether it has the right value
    if ismember(event(i).value, cfg.trialdef.eventvalue)
      % add this to the trl definition
      begsample     = event(i).sample - cfg.trialdef.prestim*hdr.Fs;
      endsample     = event(i).sample + cfg.trialdef.poststim*hdr.Fs - 1;
      offset        = -cfg.trialdef.prestim*hdr.Fs;  
      trigger       = event(i).value; % remember the trigger (=condition) for each trial
      trl(end+1, :) = [round([begsample endsample offset])  trigger]; 
    end
  end
end