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// File: math/SafeMath.sol
pragma solidity 0.5.17;
library SafeMath {
  function add(uint256 a, uint256 b) internal pure returns (uint256 c) {
    c = a + b;
    require(c >= a, "SafeMath: addition overflow");
  }
  function sub(uint256 a, uint256 b) internal pure returns (uint256 c) {
    require(b <= a, "SafeMath: subtraction overflow");
    return a - b;
  }
  function mul(uint256 a, uint256 b) internal pure returns (uint256 c) {
    if (a == 0) {
      return 0;
    }
    c = a * b;
    require(c / a == b, "SafeMath: multiplication overflow");
  }
  function div(uint256 a, uint256 b) internal pure returns (uint256 c) {
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