`instance_eval` and `class_eval` are very hairy if you pass strings (which you MUST do for some kinds of template metaprogramming). But they aren't hairy at all

`instance_eval` and `class_eval` are very hairy if you pass strings (which you MUST do for some kinds of template metaprogramming). But they aren't hairy at all if you pass blocks, where all you are doing is (essentially) setting `this`.