function _normalizeUnderlyingTokensToDebt(address underlyingToken, uint256 amount) internal view returns (uint256) {
    return amount * _underlyingTokens[underlyingToken].conversionFactor;
}
function repay(address underlyingToken, uint256 amount, address recipient) external override lock returns (uint256) {
    ...
    uint256 credit = _normalizeUnderlyingTokensToDebt(underlyingToken, actualAmount);

    // Update the recipient's debt.
    _updateDebt(recipient, -SafeCast.toInt256(credit));
    ...
