invert condition

Library/Homebrew/formula.rb

@@ -629,11 +629,10 @@ private
   end
 
   def set_instance_variable(type)
-    unless instance_variable_defined? "@#{type}"
+    return if instance_variable_defined? "@#{type}"
     class_value = self.class.send(type)
     instance_variable_set("@#{type}", class_value) if class_value
   end
-  end
 
   def method_added method
     raise 'You cannot override Formula.brew' if method == 'brew'