code: function isExtendable(...args) {
      // this is a fairly expensive call. find a way to optimize further?
      if (args.length < 1)
          return false;
  
      let baseType = typeForExtend(args[0]);
      if (!(
                 baseType === types.array
              || baseType === types.object
              || baseType === types.set
              || baseType === types.map
              || baseType === types.function)) {
          return false;
      }
      for (let i = 1; i < args.length; i++) {
          let targetType = typeForExtend(args[i]);
          if (targetType !== baseType)
              return false;
      }
      return true;
  }
docs: Internal method determining whether or not the provided arguments
  can be smashed or extended together, based on types.
  
  @param x The first item to compare.
  @param y The second item to compare.
  @param rc The running counter for comparing circular references.
  @returns {boolean} An indication as to whether or not x and y were equal.