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pragma solidity ^0.4.25;
// ----------------------------------------------
library SafeMath {
 /**
 *  Multiplies two numbers, throws on overflow.
 */
 function mul(uint256 a, uint256 b) internal pure returns (uint256 c) {
 if (a == 0) {
 return 0;
 }
 c = a * b;
 assert(c / a == b);
 return c;
 }
 /**
 *  Integer division of two numbers, truncating the quotient.
 */
 function div(uint256 a, uint256 b) internal pure returns (uint256) {
 // assert(b > 0); // Solidity automatically throws when dividing by 0
 // uint256 c = a / b;
 // assert(a == b * c + a % b); // There is no case in which this doesn't hold
 return a / b;
 }
 /**
 *  Subtracts two numbers, throws on overflow (i.e. if subtrahend is greater than minuend).
 */
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