A number of lightweight markup languages extend Markdown by implementing added features (such as tables, footnotes, definition lists, and Markdown inside HTML blocks) not available with plain Markdown syntax. In some cases, this is in order to enable conversion into more formats than HTML, for example, LaTeX, RTF, and DocBook.
Many implementations also intentionally omit support for middle word emphasis. The original Markdown implementation interprets constructs like my_long_variable as a request to emphasize "long" in the middle of a word. Many users found this confusing, so many later implementations such as PHP Markdown and Python Markdown do not implement middle word emphasis by default 
The essay "Thoughts on Markdown" stated that John Gruber (Markdown's original developer) has not responded to discussions about extensions to Markdown, and that "Markdown is changing, with or without him".
- MultiMarkdown is an implementation of Markdown oriented toward writing, which includes various extensions, including footnotes and RTF and LaTeX output.
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