Added missing space to error string

lib/util.js

@@ -270,7 +270,7 @@ function method_override(
             if ( arg_len < super_method.definition.length )
             {
                 throw new Error(
-                    "Declaration of " + name + " must be compatiable" +
+                    "Declaration of " + name + " must be compatiable " +
                         "with that of its supertype"
                 );
             }

test/test-interface-extend.js

@@ -70,3 +70,11 @@ assert.ok(
     "Interface contains defined abstract methods"
 );
 
+
+var SubType = Interface.extend( BaseType, {} );
+
+assert.ok(
+    ( SubType instanceof BaseType ),
+    "Generic interface extend method can extend from other interfaces"
+);
+