1. If an end-of-line sequence immediately follows the indentation and the indentation length is nonzero:

  a. If the indentation contains “!”, it is an error; an implementation SHOULD reject it.

"""

Why error rather than allowing it as either ignored or terminating line?

I can see how punting the question "is ! empty" simplifies the spec, but it may be a missed opportunity.

Once you prefix a block with !, it seems to me there is no reason to additionally require a comment — "! ;" (or other workarounds) — to express ignored vertical whitespace.

Isn't this the cleanest thing possible:

define long-func(x)

  let ((foo bar(x)))

  ! do stuff

  ! ...

  !

  ! more stuff

  ! ...

In any case, I didn't find this discussed in the rationale.

Beni Cherniavsky-Paskin