SafeMath
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Provides functions of mathematical operations with safety checks.

assert(bool assertion) internal
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Throws an error if the passed result is false. Used in this contract by checking mathematical expressions.

mul(uint256 a, uint256 b) internal returns (uint256)
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Multiplies two unsigned integers. Asserts that dividing the product by the non-zero multiplicand results in the multiplier.

sub(uint256 a, uint256 b) internal returns (uint256)
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Checks that b is not greater than a before subtracting.

add(uint256 a, uint256 b) internal returns (uint256)
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Checks that the result is greater than both a and b.