code: private void copyChildren(int upToChildPosition)
      {
          // (ensureRoom isn't called here, as we should always be at/behind key additions)
          if (copyFromChildPosition >= upToChildPosition)
              return;
          int len = upToChildPosition - copyFromChildPosition;
          if (len > 0)
          {
              System.arraycopy(copyFrom, getKeyEnd(copyFrom) + copyFromChildPosition, buildChildren, buildChildPosition, len);
              copyFromChildPosition = upToChildPosition;
              buildChildPosition += len;
          }
      }
docs: copies children from copyf to the builder, up to the provided index in copyf (exclusive)