Use UniswapV2Library in simple oracle constructor (cheaper than getPa…

…ir) and add a small comment

contracts/ExampleOracleSimple.sol

@@ -4,6 +4,8 @@ import '@uniswap/v2-core/contracts/interfaces/IUniswapV2Factory.sol';
 import '@uniswap/v2-core/contracts/interfaces/IUniswapV2Pair.sol';
 import '@uniswap/lib/contracts/libraries/FixedPoint.sol';
 
+import './libraries/UniswapV2Library.sol';
+
 // fixed window oracle that recomputes the average price for the entire period once every period
 // note that the price average is only guaranteed to be over at least 1 period, but may be over a longer period
 contract ExampleOracleSimple {
@@ -22,7 +24,7 @@ contract ExampleOracleSimple {
     FixedPoint.uq112x112 public price1Average;
 
     constructor(address factory, address tokenA, address tokenB) public {
-        IUniswapV2Pair _pair = IUniswapV2Pair(IUniswapV2Factory(factory).getPair(tokenA, tokenB));
+        IUniswapV2Pair _pair = IUniswapV2Pair(UniswapV2Library.pairFor(factory, tokenA, tokenB));
         pair = _pair;
         token0 = _pair.token0();
         token1 = _pair.token1();
@@ -63,6 +65,7 @@ contract ExampleOracleSimple {
         blockTimestampLast = blockTimestamp;
     }
 
+    // note this will always return 0 before update has been called successfully for the first time.
     function consult(address token, uint amountIn) external view returns (uint amountOut) {
         if (token == token0) {
             amountOut = price0Average.mul(amountIn).decode144();