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  "source": "// SPDX-License-Identifier: SEE LICENSE IN LICENSE\npragma solidity 0.6.12;\nimport \"../token/interfaces/IERC20Token.sol\";\n\ncontract TokenHandler {\n    bytes4 private constant APPROVE_FUNC_SELECTOR = bytes4(keccak256(\"approve(address,uint256)\"));\n    bytes4 private constant TRANSFER_FUNC_SELECTOR = bytes4(keccak256(\"transfer(address,uint256)\"));\n    bytes4 private constant TRANSFER_FROM_FUNC_SELECTOR = bytes4(keccak256(\"transferFrom(address,address,uint256)\"));\n\n    /**\n      * @dev executes the ERC20 token's `approve` function and reverts upon failure\n      * the main purpose of this function is to prevent a non standard ERC20 token\n      * from failing silently\n      *\n      * @param _token   ERC20 token address\n      * @param _spender approved address\n      * @param _value   allowance amount\n    */\n    function safeApprove(IERC20Token _token, address _spender, uint256 _value) internal {\n        (bool success, bytes memory data) = address(_token).call(abi.encodeWithSelector(APPROVE_FUNC_SELECTOR, _spender, _value));\n        require(success && (data.length == 0 || abi.decode(data, (bool))), 'ERR_APPROVE_FAILED');\n    }\n\n    /**\n      * @dev executes the ERC20 token's `transfer` function and reverts upon failure\n      * the main purpose of this function is to prevent a non standard ERC20 token\n      * from failing silently\n      *\n      * @param _token   ERC20 token address\n      * @param _to      target address\n      * @param _value   transfer amount\n    */\n    function safeTransfer(IERC20Token _token, address _to, uint256 _value) internal {\n       (bool success, bytes memory data) = address(_token).call(abi.encodeWithSelector(TRANSFER_FUNC_SELECTOR, _to, _value));\n        require(success && (data.length == 0 || abi.decode(data, (bool))), 'ERR_TRANSFER_FAILED');\n    }\n\n    /**\n      * @dev executes the ERC20 token's `transferFrom` function and reverts upon failure\n      * the main purpose of this function is to prevent a non standard ERC20 token\n      * from failing silently\n      *\n      * @param _token   ERC20 token address\n      * @param _from    source address\n      * @param _to      target address\n      * @param _value   transfer amount\n    */\n    function safeTransferFrom(IERC20Token _token, address _from, address _to, uint256 _value) internal {\n       (bool success, bytes memory data) = address(_token).call(abi.encodeWithSelector(TRANSFER_FROM_FUNC_SELECTOR, _from, _to, _value));\n        require(success && (data.length == 0 || abi.decode(data, (bool))), 'ERR_TRANSFER_FROM_FAILED');\n    }\n}\n",
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