Created using remix-ide: Realtime Ethereum Contract Compiler and Runtime. Load this file by pasting this gists URL or ID at https://remix.ethereum.org/#version=soljson-v0.6.10+commit.00c0fcaf.js&optimize=true&gist=

CarefulMath.sol

/* SPDX-License-Identifier: LGPL-3.0-or-later */
pragma solidity ^0.6.10;

/**
 * @title CarefulMath
 * @author Compound
 * @notice Exponential module for storing fixed-precision decimals.
 * @dev Derived from OpenZeppelin's SafeMath library
 */
abstract contract CarefulMath {
    /**
     * @notice Possible error codes that can be returned.
     */
    enum MathError { NO_ERROR, DIVISION_BY_ZERO, INTEGER_OVERFLOW, INTEGER_UNDERFLOW, MODULO_BY_ZERO }

    /**
     * @notice Adds two numbers, returns an error on overflow.
     */
    function addUInt(uint256 a, uint256 b) internal pure returns (MathError, uint256) {
        uint256 c = a + b;

        if (c >= a) {
            return (MathError.NO_ERROR, c);
        } else {
            return (MathError.INTEGER_OVERFLOW, 0);
        }
    }

    /**
     * @notice Add `a` and `b` and then subtract `c`.
     */
    function addThenSubUInt(
        uint256 a,
        uint256 b,
        uint256 c
    ) internal pure returns (MathError, uint256) {
        (MathError err0, uint256 sum) = addUInt(a, b);

        if (err0 != MathError.NO_ERROR) {
            return (err0, 0);
        }

        return subUInt(sum, c);
    }

    /**
     * @notice Integer division of two numbers, truncating the quotient.
     */
    function divUInt(uint256 a, uint256 b) internal pure returns (MathError, uint256) {
        if (b == 0) {
            return (MathError.DIVISION_BY_ZERO, 0);
        }

        return (MathError.NO_ERROR, a / b);
    }

    /**
     * @notice Returns the remainder of dividing two numbers.
     * @dev Reverts with custom message when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     */
    function modUInt(uint256 a, uint256 b) internal pure returns (MathError, uint256) {
        if (b == 0) {
            return (MathError.MODULO_BY_ZERO, 0);
        }

        return (MathError.NO_ERROR, a % b);
    }

    /**
     * @notice Multiplies two numbers, returns an error on overflow.
     */
    function mulUInt(uint256 a, uint256 b) internal pure returns (MathError, uint256) {
        if (a == 0) {
            return (MathError.NO_ERROR, 0);
        }

        uint256 c = a * b;

        if (c / a != b) {
            return (MathError.INTEGER_OVERFLOW, 0);
        } else {
            return (MathError.NO_ERROR, c);
        }
    }

    /**
     * @notice Subtracts two numbers, returns an error on overflow (i.e. if subtrahend is greater than minuend).
     */
    function subUInt(uint256 a, uint256 b) internal pure returns (MathError, uint256) {
        if (b <= a) {
            return (MathError.NO_ERROR, a - b);
        } else {
            return (MathError.INTEGER_UNDERFLOW, 0);
        }
    }
}

CarefulMathShowcase.sol

/* SPDX-License-Identifier: LGPL-3.0-or-later */
pragma solidity ^0.6.10;

import "./CarefulMath.sol";

contract CarefulMathShowcase is CarefulMath {
    function doMath(uint256 a, uint256 b) external pure returns (uint256) {
        MathError mathErr;
        uint256 result;
        (mathErr, result) = addUInt(a, b);
        require(mathErr == MathError.NO_ERROR, "BESPOKE_MATH_ERROR");
        return result;
    }
}

SafeMath.sol

/* SPDX-License-Identifier: LGPL-3.0-or-later */
pragma solidity ^0.6.10;

/**
 * @title Erc20
 * @author OpenZeppelin
 * @notice Wrappers over Solidity's arithmetic operations with added overflow
 * checks.
 *
 * Arithmetic operations in Solidity wrap on overflow. This can easily result
 * in bugs, because programmers usually assume that an overflow raises an
 * error, which is the standard behavior in high level programming languages.
 * `SafeMath` restores this intuition by reverting the transaction when an
 * operation overflows.
 *
 * Using this library instead of the unchecked operations eliminates an entire
 * class of bugs, so it's recommended to use it always.
 */
library SafeMath {
    /**
     * @dev Returns the addition of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `+` operator.
     *
     * Requirements:
     * - Addition cannot overflow.
     */
    function add(uint256 a, uint256 b) internal pure returns (uint256) {
        uint256 c = a + b;
        require(c >= a, "SafeMath: addition overflow");

        return c;
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     * - Subtraction cannot overflow.
     */
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
        return sub(a, b, "SafeMath: subtraction overflow");
    }

    /**
     * @dev Returns the subtraction of two unsigned integers, reverting with custom message on
     * overflow (when the result is negative).
     *
     * Counterpart to Solidity's `-` operator.
     *
     * Requirements:
     * - Subtraction cannot overflow.
     */
    function sub(
        uint256 a,
        uint256 b,
        string memory errorMessage
    ) internal pure returns (uint256) {
        require(b <= a, errorMessage);
        uint256 c = a - b;

        return c;
    }

    /**
     * @dev Returns the multiplication of two unsigned integers, reverting on
     * overflow.
     *
     * Counterpart to Solidity's `*` operator.
     *
     * Requirements:
     * - Multiplication cannot overflow.
     */
    function mul(uint256 a, uint256 b) internal pure returns (uint256) {
        // Gas optimization: this is cheaper than requiring 'a' not being zero, but the
        // benefit is lost if 'b' is also tested.
        // See: https://github.com/OpenZeppelin/openzeppelin-contracts/pull/522
        if (a == 0) {
            return 0;
        }

        uint256 c = a * b;
        require(c / a == b, "SafeMath: multiplication overflow");

        return c;
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     * - The divisor cannot be zero.
     */
    function div(uint256 a, uint256 b) internal pure returns (uint256) {
        return div(a, b, "SafeMath: division by zero");
    }

    /**
     * @dev Returns the integer division of two unsigned integers. Reverts with custom message on
     * division by zero. The result is rounded towards zero.
     *
     * Counterpart to Solidity's `/` operator. Note: this function uses a
     * `revert` opcode (which leaves remaining gas untouched) while Solidity
     * uses an invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     * - The divisor cannot be zero.
     */
    function div(
        uint256 a,
        uint256 b,
        string memory errorMessage
    ) internal pure returns (uint256) {
        // Solidity only automatically asserts when dividing by 0
        require(b > 0, errorMessage);
        uint256 c = a / b;
        // assert(a == b * c + a % b); // There is no case in which this doesn't hold

        return c;
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     * - The divisor cannot be zero.
     */
    function mod(uint256 a, uint256 b) internal pure returns (uint256) {
        return mod(a, b, "SafeMath: modulo by zero");
    }

    /**
     * @dev Returns the remainder of dividing two unsigned integers. (unsigned integer modulo),
     * Reverts with custom message when dividing by zero.
     *
     * Counterpart to Solidity's `%` operator. This function uses a `revert`
     * opcode (which leaves remaining gas untouched) while Solidity uses an
     * invalid opcode to revert (consuming all remaining gas).
     *
     * Requirements:
     * - The divisor cannot be zero.
     */
    function mod(
        uint256 a,
        uint256 b,
        string memory errorMessage
    ) internal pure returns (uint256) {
        require(b != 0, errorMessage);
        return a % b;
    }
}

SafeMathShowcase.sol

/* SPDX-License-Identifier: LGPL-3.0-or-later */
pragma solidity ^0.6.10;

import "./SafeMath.sol";

contract SafeMathShowcase {
    using SafeMath for uint256;

    function doMath(uint256 a, uint256 b) external pure returns (uint256) {
        uint256 result = a.add(b);
        return result;
    }
}