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+# spellcheck
+
+This filter checks the spelling of words in the body of the
+document (omitting metadata).  The external program `aspell` is
+used for the checking, and must be present in the path.
+
+Why use this instead of just running `aspell` on the
+document's source?  Because this filter is sensitive to
+the semantics of the document in ways that `aspell` is
+not:
+
+- Material in code spans, raw HTML, URLs in links,
+  and math is not spell-checked, eliminating a big
+  class of false positives.
+
+- The filter is sensitive to the `lang` specified in
+  the document's metadata; this will be treated as the
+  default language for the document.
+
+- It is also sensitive to `lang` attributes on native
+  divs and spans. Thus, for example, in an English
+  document, `[chevaux]{lang=fr}` will not be registered
+  as a spelling error.
+
+To run it,
+
+    pandoc --lua-filter spellcheck.lua sample.md
+
+A list of misspelled words (or at any rate, words not
+in the appropriate dictionary) will be printed to stdout.
+If the word is in a div or span with a non-default `lang`
+attribute, the relevant language will be indicated in
+brackets after the word, separated by a tab.
+
+To add words to the list for a language, you can add files
+with names `.aspell.LANG.pws` in your home directory.  Example:
+
+```
+% cat ~/.aspell.en.pws
+personal_ws-1.1 en 0
+goopy
+```