code: void setFirstKey(ByteArraySegment newKey) {
          Preconditions.checkState(getCount() > 0, "BTreePage is empty. Cannot set first key.");
          Preconditions.checkArgument(newKey.getLength() == this.config.getKeyLength(), "Incorrect key length.");
          Preconditions.checkArgument(KEY_COMPARATOR.compare(newKey, getKeyAt(0)) <= 0,
                  "Replacement first Key must be smaller than or equal to the existing first key.");
  
          this.data.copyFrom(newKey, 0, newKey.getLength());
      }
docs: Updates the first PageEntry's key to the given value.
  
  @param newKey A ByteArraySegment representing the replacement value for the first key. This must be smaller than
  or equal to the current value of the first key (using KEY_COMPARATOR).