bug: How to deal with {{<code>}} blocks

Notice how I mis-copy/pasted and I "end a block" before it started. Below this block, I end it a second time. No warnings either time, no step5 emitted by the transformer. Ideally, the code would say "hey dude you ended something before you started it" and fail.

Start by locating the transformer code that handles {{}} blocks and reproduce the issue with a block ending before it starts, followed by a second end. Check the existing behavior around the missing step5 output. Done means the malformed block is reported as an error or warning and the transformer’s behavior is covered by a regression test.

Written by the indexing model from the issue text.