# File lib/ruote/exp/fe_subprocess.rb, line 201
    def invoke_engine_participant

      atts = tree[1]

      if ref = atts.find { |k, v| v.nil? }
        ref = ref.first
        atts.delete(ref)
      end
      atts['pdef'] = atts['ref'] || ref
      atts['ref'] = atts.delete('engine')

      @h['name'] = 'participant'
      Ruote::Exp::ParticipantExpression.new(@context, @h).apply
    end