improve PullPaymentCapable

contracts/PullPayment.sol

@@ -13,11 +13,16 @@ contract PullPayment {
   }
 
   // withdraw accumulated balance, called by payee
-  function withdrawPayments() external {
-    uint payment = payments[msg.sender];
-    payments[msg.sender] = 0;
-    if (!msg.sender.send(payment)) {
-      payments[msg.sender] = payment;
+  function withdrawPayments() {
+    address payee = msg.sender;
+    uint payment = payments[payee];
+    
+    if (payment == 0) throw;
+    if (this.balance < payment) throw;
+
+    payments[payee] = 0;
+    if (!payee.send(payment)) {
+      throw;
     }
   }
 }

contracts/examples/PullPaymentExample.sol

@@ -1,11 +0,0 @@
-pragma solidity ^0.4.0;
-
-import '../PullPayment.sol';
-
-// Example class using PullPayment
-contract PullPaymentExample is PullPayment {
-  // test helper function to call asyncSend
-  function callSend(address dest, uint amount) external {
-    asyncSend(dest, amount);
-  }
-}

contracts/test-helpers/PullPaymentMock.sol

@@ -4,7 +4,8 @@ import '../PullPayment.sol';
 // mock class using PullPayment
 contract PullPaymentMock is PullPayment {
   // test helper function to call asyncSend
-  function callSend(address dest, uint amount) external {
+  function callSend(address dest, uint amount) {
     asyncSend(dest, amount);
   }
+
 }