controlled delegation and set default delegate

contracts/democracy/delegation/DelegationAbstract.sol

@@ -1,13 +1,14 @@
 pragma solidity >=0.5.0 <0.6.0;
 
 import "../../deploy/InstanceAbstract.sol";
+import "../../common/Controlled.sol";
 import "./Delegation.sol";
 
 /**
  * @title DelegationAbstract
  * @author Ricardo Guilherme Schmidt (Status Research & Development GmbH). 
  */
-contract DelegationAbstract is InstanceAbstract, Delegation {
+contract DelegationAbstract is InstanceAbstract, Controlled, Delegation {
     struct DelegateSet {
         uint96 fromBlock; //when this was updated
         address to; //who recieved this delegaton

contracts/democracy/delegation/DelegationBase.sol

@@ -17,7 +17,7 @@ contract DelegationBase is DelegationAbstract {
     }
 
     /** 
-     * @notice Changes the delegation of `msg.sender` to `_to`. if _to 0x00: delegate to self. 
+     * @notice Changes the delegation of `msg.sender` to `_to`. 
      *         In case of having a parent proxy, if never defined, fall back to parent proxy. 
      *         If once defined and want to delegate to parent proxy, set `_to` as parent address. 
      * @param _to To what address the caller address will delegate to.
@@ -26,6 +26,16 @@ contract DelegationBase is DelegationAbstract {
         updateDelegate(msg.sender, _to);
     }
 
+    /** 
+     * @notice Changes the delegation of `address(0)` to `_to`.
+     *         By default accounts delegate to `address(0)`. 
+     *         Therefore the delegate of `address(0)` is the default delegate of all accounts.
+     * @param _defaultDelegate default delegate address
+     */
+    function setDefaultDelegate(address _defaultDelegate) external onlyController {
+        updateDelegate(address(0), _defaultDelegate);
+    }
+
     /**
      * @notice Reads `_who` configured delegation in this level, 
      *         or from parent level if `_who` never defined/defined to parent address.