Since the author admittedly made up the term "complete expression" whose definition mirrors what I would expect for just "expression". He also used the word "s

Since the author admittedly made up the term "complete expression" whose definition mirrors what I would expect for just "expression". He also used the word "subexpression" to describe parts of something that would otherwise be called a "statement". Though I think his vocabulary is a little confusing, after re-reading I think I get what he's actually complaining about. It is really awkward for a <%= %> tag to not actually be a self-contained, parse-able expression. On the other hand, you'd really rather have things like form_for return the text to print, not just print it (thus allowing the use of the <%= %> instead of <% %>). It's just one of those annoying situations where attempting to be pure and elegant results in an ugly kludge in the real world.