mmark-filter is a markdown filter. It takes in a markdown file (possibly on standard input) and translates the input according to a bunch of filters (implemented as plugins).
Currently these plugins exist:
noop, a plugin that does nothing.
emph, a plugin that replaces
XXemphasisXX, added mostly as an example.
exec, a plugin the runs a command using the codeblock contents and replaces it with an image.
rot13, a plugin that rot13's all text, but leaves all other elements alone.
protocol, a plugin that runs protocol on the contents of a codeblock and replaces the codeblock's content with its output. Comes in handy when writting IETF drafts.