Fenced divs are containers for sequences of blocks, to
which an attribute can be attached.
A fenced div begins with an opening fence: a line with three or
more consecutive `:` characters, followed by an attribute
specifier, followed by optional whitespace and the end of the
line.
It ends with a closing fence: a line beginning with at
least as many consecutive `:` characters as in the opening
fence, followed by optional whitespace and the end of the line.
If the end of the input (or enclosing block) is encountered
before a closing fence, the fenced div is implicitly closed.
```````````````````````````````` example
::: {#bar .foo}
Hi
> A block quote.
:::
.
````````````````````````````````
```````````````````````````````` example
:::: {#bar .foo}
Hi
> A block quote.
:::::::::::::::::::::::::
.
````````````````````````````````
Fenced divs may be nested.
```````````````````````````````` example
::: {#bar .foo}
Hi
::: {.baz}
> A block quote.
:::
:::
.
````````````````````````````````
A fenced div can interrupt a paragraph, without
an intervening blank line.
```````````````````````````````` example
Paragraph text
::: {#bar .foo}
Hi
:::
.
Paragraph text
````````````````````````````````
A fenced div *must* have attributes.
```````````````````````````````` example
:::
Hi
:::
.
:::
Hi
:::
````````````````````````````````
The closing fence must be at leats as long as the opening
fence.
```````````````````````````````` example
::::: {.foo}
Hi
:::
::::::
.
````````````````````````````````
If the end of the input (or enclosing block) is encountered
before a closing fence, the fenced div is implicitly closed.
```````````````````````````````` example
> ::: {.foo}
> Hi
.
````````````````````````````````
Instead of a normal attribute specifier in curly braces,
a single bare word may be used; it will be treated as a
"class" attribute:
```````````````````````````````` example
::: c_d
Hi
:::
.
````````````````````````````````