The lengths Rails goes to to enable this:
```erb
<%= form_for(Widget) do %>
<h1>Inside form</h1>
<% end %>
```
is amazing. It has to subclass Erubi::Engine and override most methods.
Erubi/ERB treat <%= xxx %> as an expression and wrap it in parens `_buf << (xxx)`
Thus, `_buf << (form_for(Widget) do)` is a syntax error by default.
Making something like this work in Erubi on its own is not very easy and so this is a great example of how much work Rails does to provide an ergonomic API.