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library.sol

pragma solidity ^0.4.0;

library IntExtended {
    
    function increment(uint _self) public pure returns (uint) {
        return _self + 1;
    }
    
    function decrement(uint _self) public pure returns (uint) {
        return _self - 1;
    }
    
    function incrementValue(uint _self, uint _value) public pure returns (uint) {
        return _self + _value;
    }
    
    function decrementValue(uint _self, uint _value) public pure returns (uint) {
        return _self - _value;
    }
}

myfirstcontract.sol

pragma solidity ^0.4.0;

interface Regulator {
    
    function checkValue(uint amount) external returns (bool);
    
    function loan() external returns (bool);
    
}

contract Bank is Regulator {
    uint private Value;
    address private Owner;
    
    modifier ownerAccess {
        require(Owner == msg.sender);
        _;
    }
    
    constructor(uint amount) public {
        Value = amount;
        Owner = msg.sender;
    }
    
    function deposit(uint amount) public ownerAccess {
        Value += amount;
    }
    
    function withdraw(uint amount) public ownerAccess {
        if (checkValue(amount)){
            Value -= amount;
        }
    }
    
    function balance() public view returns (uint) {
        return Value;
    }
    
    function checkValue(uint amount) public view returns (bool) {
        return Value >= amount;
    }
    
    function loan() public view returns (bool) {
        return Value > 0;
    }
}

contract MyFirstContract is Bank(10) {
    string private Name;
    uint private Age;
    
    function setName(string newName) public {
        Name = newName;
    }
    
    function getName() public view returns (string) {
        return Name;
    }
    
    function setAge(uint newAge) public {
        Age = newAge;
    }
    
    function getAge() public view returns (uint) {
        return Age;
    }
}

testLibrary.sol

pragma solidity ^0.4.0;

import "browser/library.sol";

contract TestLibrary {
    using IntExtended for uint;
    
    function testIncrement(uint _base) public pure returns (uint) {
        return _base.increment();
    }
    
    function testDecrement(uint _base) public pure returns (uint) {
        return _base.decrement();
    }
    
    function testIncrementValue(uint _base, uint _value) public pure returns (uint) {
        return _base.incrementValue(_value);
    }
    
    function testDecrementValue(uint _base, uint _value) public pure returns (uint) {
        return _base.decrementValue(_value);
    }
    
}