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