  function transferFrom(
    address from,
    address to,
    uint256 amount
  ) external virtual nonReentrant sphereXGuardExternal returns (bool) {
    uint256 allowed = allowance[from][msg.sender];

    // Saves gas for unlimited approvals.
    if (allowed != type(uint256).max) {
      uint256 newAllowance = allowed - amount;
      _approve(from, msg.sender, newAllowance);
    }

    _transfer(from, to, amount, 0x64);

    return true;
  }
