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library.sol

pragma solidity ^0.4.0;

//keyword library
library IntExtended{
    function increment(uint _base) returns(uint){
        return _base+1;
    }
    function decrement(uint _base) returns(uint){
        return _base-1;
    }
    function incrementByValue(uint _base, uint _value) returns(uint){
        return _base+_value;
    }
    function decrementByValue(uint _base, uint _value) returns(uint){
        return _base-_value;
    }
}

testLibrary.sol

pragma solidity ^0.4.0;

//import statement
import "browser/library.sol";

contract testLibrary{
    // use <libraryName> for <extendedClass>
    using IntExtended for uint;
    
    function testIncrement(uint _base) returns(uint){
        //Because it's a library functon for uint the first parameter
        //is used as the base object here.
        //It could be written like this also:
        //reutrn IntExtended.increment(_base);
        return _base.increment();
    }
    
    function testIncrementByValue(uint _base, uint _value) returns(uint){
        return _base.incrementByValue(_value);
    }
}