# File lib/ruote/svc/dollar_sub.rb, line 126
      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' # deprecated in 2.1.12
        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)

        # stage 0

        v = lookup(pr[0, 1], k)

        return v if v != nil

        # stage 1

        return '' if pr.size < 2

        lookup(pr[1, 1], k)
      end