Obviously the Spec would need to force a space after the :: symbol for nesting (in contrast to >> generating a nested blockquote.
:: outer div block
:: :: nested div block
:: with laziness
::
:: outer div block
with laziness
How would you define the upper bar?
I see a difficulty coming up if we want to have the combined variant. You would need to use at least three or more colons i guess: :::
::: WARNING
no left border
:::