A paragraph containing only two colons indicates that the following indented or quoted text is a literal block (i.e. where no markup processing is done), which are used e.g. to illustrate plaintext markup.
Whitespace, newlines, blank lines and all kinds of markup (like *this* or \this) is preserved by literal blocks. The paragraph containing only '::' will be omitted from the result.
The :: may be tracked onto the very end of any paragraph. The :: will be omitted it is preceded by whitespace. The :: will be converted to a single colon if preceded by text, like this:
It's very convenient to use this form.
Literal blocks end when text returns to the preceding paragraph's indentation. This means that something like this is possible:
We start here and continue here and end here.
Per-line quoting can also be used on unindented literal blocks:
> Useful for quotes from email and > for Haskell literate programming.