# File lib/ruote/part/engine_participant.rb, line 175
    def determine_tree(fexp, params)

      pdef = params['def'] || params['pdef'] || params['tree']

      tree = Ruote.lookup_subprocess(fexp, pdef)

      raise(
        "couldn't find process definition behind '#{pdef}'"
      ) unless tree

      tree.last
    end