docs: function parseFieldOperator(field, operator, operand) {
      if(operator === '$elemMatch') {
          var elemMatchInfo = parseElemMatch(operand)
          var innerParts = elemMatchInfo.parts
          var implicitField = elemMatchInfo.implicitField
      } else if(operator === '$not') {
          var innerParts = parseNot(field, operand)
      } else {
          var innerParts = []
      }
      return new Part(field, operator, operand, innerParts, implicitField)
  }