# File lib/ruote/util/dollar.rb, line 78
    def [] (key)

      return @fexp.fei.to_storage_id if key == 'fei'
      return @fexp.fei.wfid if key == 'wfid'
      return @fexp.fei.sub_wfid if key == 'sub_wfid'
      return @fexp.fei.expid if key == 'expid'

      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