code: synchronized void remove(long size, int generation) {
          Preconditions.checkArgument(size >= 0, "size must be a non-negative number");
          this.totalSize -= size;
          if (this.totalSize < 0) {
              this.totalSize = 0;
          }
  
          removeFromGeneration(generation);
      }
docs: Records the removal of an element of the given size from the given generation.
  
  @param size       The size of the element to remove.
  @param generation The generation of the element to remove.