Moved EnumerableSet to utils.

contracts/utils/EnumerableSet.sol

@@ -0,0 +1,72 @@
+pragma solidity ^0.5.10;
+
+
+/**
+ * @title EnumerableSet
+ * @dev Data structure
+ * @author Alberto Cuesta Cañada
+ */
+library EnumerableSet {
+
+    event ValueAdded(address value);
+    event ValueRemoved(address value);
+
+    struct Set {
+        // Position of the value in the `values` array, plus 1 because index 0
+        // means a value is not in the set.
+        mapping (address => uint256) index;
+        address[] values;
+    }
+
+    /**
+     * @dev Add a value.
+     */
+    function add(Set storage set, address value)
+        internal
+    {
+        if (!contains(set, value)){
+            set.index[value] = set.values.push(value);
+            emit ValueAdded(value);
+        }
+    }
+
+    /**
+     * @dev Remove a value.
+     */
+    function remove(Set storage set, address value)
+        internal
+    {
+        if (contains(set, value)) {
+            set.values[set.index[value] - 1] = set.values[set.values.length - 1];
+            set.values.pop();
+            delete set.index[value];
+            emit ValueRemoved(value);
+        }
+    }
+
+    /**
+     * @dev Returns true if the value is in the set.
+     */
+    function contains(Set storage set, address value)
+        internal
+        view
+        returns (bool)
+    {
+        return set.index[value] != 0;
+    }
+
+    /**
+     * @dev Return an array with all values in the set.
+     */
+    function enumerate(Set storage set)
+        internal
+        view
+        returns (address[] memory)
+    {
+        address[] memory output = new address[](set.values.length);
+        for (uint256 i; i < set.values.length; i++){
+            output[i] = set.values[i];
+        }
+        return output;
+    }
+}