code: function readTertiaryExpr(walker) {
      var conditional = readLogicalORExpr(walker);
      walker.goUntil();
  
      if (walker.currentCode() === 63) { // ?
          walker.go(1);
          var yesExpr = readTertiaryExpr(walker);
          walker.goUntil();
  
          if (walker.currentCode() === 58) { // :
              walker.go(1);
              return {
                  type: ExprType.TERTIARY,
                  segs: [
                      conditional,
                      yesExpr,
                      readTertiaryExpr(walker)
                  ]
              };
          }
      }
  
      return conditional;
  }
docs: 读取三元表达式
  
  @param {Walker} walker 源码读取对象
  @return {Object}