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.