it('constructs string for lower priority prefix operator from higher priority...', () => {
      const lowPriorityPO = new PrefixOperator(simplePO)
      lowPriorityPO.priority += 1
      lowPriorityPO.keyword = 'lowerPO'
      expect(lowPriorityPO.toString()).to.equal('lowerPO simplePO selector')