function _removeTokenFromOwnerEnumeration(address from, uint256 tokenId)
  private
{
  // To prevent a gap in from's tokens array, we store the last token in the index of the token to delete, and
  // then delete the last slot (swap and pop).

  uint256 lastTokenIndex = _ownedTokens[from].length.sub(1);
  uint256 tokenIndex = _ownedTokensIndex[tokenId];

  // When the token to delete is the last token, the swap operation is unnecessary
  if (tokenIndex != lastTokenIndex) {
    uint256 lastTokenId = _ownedTokens[from][lastTokenIndex];

    _ownedTokens[from][tokenIndex] = lastTokenId; // Move the last token to the slot of the to-delete token
    _ownedTokensIndex[lastTokenId] = tokenIndex; // Update the moved token's index
  }

  // This also deletes the contents at the last position of the array
  _ownedTokens[from].length--;

  // Note that _ownedTokensIndex[tokenId] hasn't been cleared: it still points to the old slot (now occupied by
  // lastTokenId, or just over the end of the array if the token was the last one).
}