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102// As _anyone_ can call `setTier` we require that `msg.sender` and
103// `account_` are the same if the end tier is not an improvement.
104// Anyone can increase anyone else's tier as the `msg.sender` is
105// responsible to pay the difference.
106if (endTier_ <= startTier_) {
107    require(msg.sender == account_, "DELEGATED_TIER_LOSS");
108}
109
110uint256[8] memory tierValues_ = tierValues();
111
112// Handle the erc20 transfer.
113// Convert the start tier to an erc20 amount.
114uint256 startValue_ = tierToValue(tierValues_, startTier_);
115// Convert the end tier to an erc20 amount.
116uint256 endValue_ = tierToValue(tierValues_, endTier_);
117
118// Short circuit if the values are the same for both tiers.
119if (endValue_ == startValue_) {
120    return;
121}
122if (endValue_ > startValue_) {
123    // Going up, take ownership of erc20 from the `msg.sender`.
124    erc20.safeTransferFrom(
125        msg.sender,
126        address(this),
127        endValue_.saturatingSub(startValue_)
128    );
129} else {
130    // Going down, process a refund for the tiered account.
131    erc20.safeTransfer(account_, startValue_.saturatingSub(endValue_));
132}

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118// Short circuit if the values are the same for both tiers.
119if (endValue_ == startValue_) {
120    return;
121}
122if (endValue_ > startValue_) {
123    // Going up, take ownership of erc20 from the `msg.sender`.
124    erc20.safeTransferFrom(
125        msg.sender,
126        address(this),
127        endValue_.saturatingSub(startValue_)
128    );
129} else {
130    // Going down, process a refund for the tiered account.
131    erc20.safeTransfer(account_, startValue_.saturatingSub(endValue_));
132}