diff --git a/inner-parens-to-brackets/LICENSE.txt b/inner-parens-to-brackets/LICENSE.txt
new file mode 100644
index 0000000..551b50b
--- /dev/null
+++ b/inner-parens-to-brackets/LICENSE.txt
@@ -0,0 +1,21 @@
+MIT License
+
+Copyright © 2024 Bastien Dumont
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
diff --git a/inner-parens-to-brackets/README.md b/inner-parens-to-brackets/README.md
new file mode 100644
index 0000000..0a4ab7a
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+++ b/inner-parens-to-brackets/README.md
@@ -0,0 +1,4 @@
+This filter converts nested parentheses to brackets (and conversely).
+
+I use it mostly to post-process citeproc output.
+
diff --git a/inner-parens-to-brackets/inner-parens-to-brackets.lua b/inner-parens-to-brackets/inner-parens-to-brackets.lua
new file mode 100644
index 0000000..124666e
--- /dev/null
+++ b/inner-parens-to-brackets/inner-parens-to-brackets.lua
@@ -0,0 +1,91 @@
+-- inner-parens-to-brackets.lua
+-- A Pandoc Lua filter that converts nested parentheses to brackets (and conversely).
+-- Copyright 2024 Bastien Dumont (bastien.dumont [at] posteo.net)
+-- This file is under the MIT License: see LICENSE for more details.
+
+
+local previously_signalled = {}
+
+local function signal_if_new(id, msg)
+  if not previously_signalled[id] then
+    io.stdout:write('INFO: ' .. msg .. '\n')
+    previously_signalled[id] = true
+  end
+end
+
+function Para(para)
+  local whole_para = pandoc.utils.stringify(para)
+  local parens_trace = {}
+  local i_str = 0
+  local isolate_parens = {
+    -- In case there are several level of parentheses in the same Str object
+    -- (could theoretically happen due to non-breaking spaces).
+    -- Also simplifies check_and_replace() below.
+    Str = function(str)
+      local text = str.text
+      if string.match(text, '[][()]') then
+        local substrings = {}
+        for a, b, c in string.gmatch(text, '([^][()]*)([][()]?)([^][()]*)') do
+          for _, sub in ipairs({ a, b, c }) do
+            if sub ~= '' then table.insert(substrings, pandoc.Str(sub)) end
+          end
+        end
+        return substrings
+      end
+    end
+  }
+  local check_and_replace = {
+    traverse = 'topdown',
+    Str = function(str)
+      i_str = i_str + 1
+      local text = str.text
+      if text == '(' then
+        if #parens_trace > 0 then
+          if parens_trace[#parens_trace] == '(' then
+            str.text = '['
+            signal_if_new(whole_para .. i_str,
+                          'Replacing a left parenthesis with a bracket ' ..
+                          'in the following item: ' .. whole_para)
+          end
+        end
+        table.insert(parens_trace, str.text)
+      elseif text == ')' then
+        if #parens_trace > 0 then
+          if parens_trace[#parens_trace] == '[' then
+            str.text = ']'
+            signal_if_new(whole_para .. i_str,
+                          'Replacing a right parenthesis with a bracket ' ..
+                          'in the following item: ' .. whole_para)
+          end
+        end
+        table.remove(parens_trace)
+      elseif text == '[' then
+        if #parens_trace > 0 then
+          if parens_trace[#parens_trace] == '[' then
+            str.text = '('
+            signal_if_new(whole_para .. i_str,
+                          'Replacing a left bracket with a parenthesis ' ..
+                          'in the following item: ' .. whole_para)
+          end
+        end
+        table.insert(parens_trace, str.text)
+      elseif text == ']' then
+        if #parens_trace > 0 then
+          if parens_trace[#parens_trace] == '(' then
+            str.text = ')'
+            signal_if_new(whole_para .. i_str,
+                          'Replacing a right bracket with a parenthesis ' ..
+                          'in the following item: ' .. whole_para)
+          end
+        end
+        table.remove(parens_trace)
+      end
+      return str
+    end
+  }
+  local processed_para = para:walk(isolate_parens):walk(check_and_replace)
+  if #parens_trace > 0 then
+    io.stdout:write('WARNING: Unbalanced parentheses or brackets found in ' .. whole_para .. '\n')
+  end
+  return processed_para
+end