Simplify and reorganize Lua filter introduction

doc/lua-filters.md

@@ -6,34 +6,19 @@ date: 'January 10, 2020'
 title: Pandoc Lua Filters
 ---
 
-# Introduction
+Create custom outputs with pandoc's embedded Lua engine.
 
-Pandoc has long supported filters, which allow the pandoc
-abstract syntax tree (AST) to be manipulated between the parsing
-and the writing phase. [Traditional pandoc
-filters](https://pandoc.org/filters.html) accept a JSON
-representation of the pandoc AST and produce an altered JSON
-representation of the AST. They may be written in any
-programming language, and invoked from pandoc using the
-`--filter` option.
-
-Although traditional filters are very flexible, they have a
-couple of disadvantages. First, there is some overhead in
-writing JSON to stdout and reading it from stdin (twice, once on
-each side of the filter). Second, whether a filter will work
-will depend on details of the user's environment. A filter may
-require an interpreter for a certain programming language to be
-available, as well as a library for manipulating the pandoc AST
-in JSON form. One cannot simply provide a filter that can be
-used by anyone who has a certain version of the pandoc
-executable.
-
-Starting with version 2.0, pandoc makes it possible to write
-filters in Lua without any external dependencies at all. A Lua
+## Introduction
+
+With Lua filters, you can write Pandoc filters without any
+external dependencies. Besides the simpler set-up, Lua filters are
+generally faster and can access utility functions to manipulate
+document elements. 
+
+Since Pandoc 2.0, the pandoc executable has a built-in Lua
 interpreter (version 5.4) and a Lua library for creating pandoc
-filters is built into the pandoc executable. Pandoc data types
-are marshaled to Lua directly, avoiding the overhead of writing
-JSON to stdout and reading it from stdin.
+filters. Pandoc data types are marshaled to Lua directly, avoiding
+the overhead of writing JSON to stdout and reading it from stdin.
 
 Here is an example of a Lua filter that converts strong emphasis
 to small caps:
@@ -62,17 +47,31 @@ replace it with a SmallCaps element with the same content.
 To run it, save it in a file, say `smallcaps.lua`, and invoke
 pandoc with `--lua-filter=smallcaps.lua`.
 
+## Why Lua filters over JSON?
+
+[JSONfilters](https://pandoc.org/filters.html) accept a JSON
+representation of the pandoc AST and produce an altered JSON
+representation of the AST. They may be written in any programming
+language, and invoked from pandoc using the `--filter` option.
+
+ However, JSON filters have limitations:
+
+ - Writing JSON to stdout and reading it from stdin (twice, once
+on each side of the filter) is inefficient.
+- External dependencies vary between users, and universal JSON
+  filters are not possible.
+
 Here's a quick performance comparison, converting the pandoc
-manual (MANUAL.txt) to HTML, with versions of the same JSON
-filter written in compiled Haskell (`smallcaps`) and interpreted
-Python (`smallcaps.py`):
-
-  Command                                 Time
-  --------------------------------------- -------
-  `pandoc`                                1.01s
-  `pandoc --filter ./smallcaps`           1.36s
-  `pandoc --filter ./smallcaps.py`        1.40s
-  `pandoc --lua-filter ./smallcaps.lua`   1.03s
+manual (MANUAL.txt) to HTML, with versions of the same JSON filter
+written in compiled Haskell (`smallcaps`) and interpreted Python
+(`smallcaps.py`): 
+
+    Command                                 Time
+    --------------------------------------- -------
+    `pandoc`                                1.01s
+    `pandoc --filter ./smallcaps`           1.36s
+    `pandoc --filter ./smallcaps.py`        1.40s
+    `pandoc --lua-filter ./smallcaps.lua`   1.03s
 
 As you can see, the Lua filter avoids the substantial overhead
 associated with marshaling to and from JSON over a pipe.