Allow to change flap and flop addresses in the vow

src/test/vow.t.sol

@@ -105,6 +105,26 @@ contract VowTest is DSTest {
         vow.heal(rad(wad));
     }
 
+    function test_change_flap_flop() public {
+        Flap newFlap = new Flap(address(vat), address(gov));
+        Flop newFlop = new Flop(address(vat), address(gov));
+
+        newFlap.rely(address(vow));
+        newFlop.rely(address(vow));
+
+        assertEq(vat.can(address(vow), address(flap)), 1);
+        assertEq(vat.can(address(vow), address(newFlap)), 0);
+
+        vow.file('flapper', address(newFlap));
+        vow.file('flopper', address(newFlop));
+
+        assertEq(address(vow.flapper()), address(newFlap));
+        assertEq(address(vow.flopper()), address(newFlop));
+
+        assertEq(vat.can(address(vow), address(flap)), 0);
+        assertEq(vat.can(address(vow), address(newFlap)), 1);
+    }
+
     function test_flog_wait() public {
         assertEq(vow.wait(), 0);
         vow.file('wait', uint(100 seconds));

src/vow.sol

@@ -36,6 +36,7 @@ contract VatLike {
     function sin (address) external view returns (uint);
     function heal(uint256) external;
     function hope(address) external;
+    function nope(address) external;
 }
 
 contract Vow is LibNote {
@@ -97,6 +98,16 @@ contract Vow is LibNote {
         else revert("Vow/file-unrecognized-param");
     }
 
+    function file(bytes32 what, address data) external note auth {
+        if (what == "flapper") {
+            vat.nope(address(flapper));
+            flapper = FlapLike(data);
+            vat.hope(data);
+        }
+        else if (what == "flopper") flopper = FlopLike(data);
+        else revert("Vow/file-unrecognized-param");
+    }
+
     // Push to debt-queue
     function fess(uint tab) external note auth {
         sin[now] = add(sin[now], tab);