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 optionally by
a class name and optionally whitespace.
It ends with a closing fence: a line beginning with three
or more consecutive `:` characters, followed by optional
whitespace and the end of the line. The number of `:` characters
in the closing fence must be at least the number in the opening fence.
If the end of the input (or enclosing block) is encountered
before a closing fence, the fenced div is implicitly closed.
```
:::::::::: foo
Hi
> A block quote.
:::::::::::
.
```
```
{#bar .foo}
:::
Hi
> A block quote.
:::::::::::::
.
```
Fenced divs may be nested.
```
{#bar .foo}
::::
Hi
::: baz
> A block quote.
:::
::::
.
```
A fenced div cannot interrupt a paragraph, without
an intervening blank line.
```
Paragraph text
::::
Hi
::::
.
Paragraph text
::::
Hi
::::
```
A fenced div need not have attributes or a class name.
```
::::
Hi
::::
.
```
The closing fence must be at least as long as the opening fence.
```
::::::::: foo
Hi
::::
.
```
If the end of the input (or enclosing block) is encountered
before a closing fence, the fenced div is implicitly closed.
```
> :::: foo
> Hi
.
```
````
::: outer
```
:::
```
:::
.
````