restruct contract repo

contracts/helper/ConstFeeRateModel.sol

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+/*
+
+    Copyright 2020 DODO ZOO.
+    SPDX-License-Identifier: Apache-2.0
+
+*/
+
+pragma solidity 0.6.9;
+pragma experimental ABIEncoderV2;
+
+import {IFeeRateModel} from "../intf/IFeeRateModel.sol";
+
+contract ConstFeeRateModel is IFeeRateModel {
+    uint256 public _FEE_RATE_;
+
+    constructor(uint256 feeRate) public {
+        feeRate = _FEE_RATE_;
+    }
+
+    function getFeeRate(uint256) external override view returns (uint256) {
+        return _FEE_RATE_;
+    }
+}