Add [fence-effects=""] to stop effect propagation

[syg]

This allows for manual fencing where needed, like in GeneratorStart.

[bakkot]

These are intended to be used only at the beginning of steps, yeah? We already have a different mechanism for metadata about steps, namely ids (which look like (1. [id="foo"] Step.); I'd prefer to reuse that (as in 1. [fences-effects="user-code"] Step.).

This would require a change to ecmarkdown; I can make that change if this approach sounds good to you.

[syg]

These are intended to be used only at the beginning of steps, yeah? We already have a different mechanism for metadata about steps, namely ids (which look like (1. [id="foo"] Step.); I'd prefer to reuse that (as in 1. [fences-effects="user-code"] Step.).

Exactly right.

This would require a change to ecmarkdown; I can make that change if this approach sounds good to you.

I admit the only reason I did it this way is because I didn't want to touch ecmarkdown. If you're offering, doing it in ecmarkdown would be ideal.

[bakkot]

Rebased on #392 and switched to the syntax suggested above.