def [](key)
return @fexp.fei.to_storage_id if key == 'fei'
return @fexp.fei.wfid if key == 'wfid'
return @fexp.fei.subid if key == 'subid'
return @fexp.fei.subid if key == 'sub_wfid'
return @fexp.fei.expid if key == 'expid'
return @fexp.fei.engine_id if key == 'engine_id'
return @fexp.fei.mnemo_id if key == 'mnemo_id'
return @workitem['fields']['__tags__'] if key == 'tags'
return (@workitem['fields']['__tags__'] || []).last if key == 'tag'
return (@workitem['fields']['__tags__'] || []).join('/') if key == 'full_tag'
pr, k = extract_prefix(key)
v = lookup(pr[0, 1], k)
return v if v != nil
return '' if pr.size < 2
lookup(pr[1, 1], k)
end