diff --git a/README.md b/README.md
index 48e5c84..83f6c8b 100644
--- a/README.md
+++ b/README.md
@@ -6,7 +6,7 @@ process of creating works of literature][1]".
 
 ## Requirements
 
--   [Pandoc]
+-   [Pandoc] v3
 
 ## Install
 
diff --git a/dist/lin.lua b/dist/lin.lua
index 0de774e..85727c6 100644
--- a/dist/lin.lua
+++ b/dist/lin.lua
@@ -6302,10 +6302,14 @@ function os.isunix()
   return os.uname() ~= "windows"
 end
 
--- Changes newlines so everything is in one line
--- @return string: String with formatted newlines as literal "\n"
-function string:linearize()
-  return self:gsub("\n", "\\\\n")
+-- Gets OS language
+-- @return string, string, string: Language, locale and encoding
+function os.lang()
+  local lang = os.getenv("LANG")
+  if lang ~= nil then
+    return lang:match("(%w%w)_?(%w?%w?)%.?(.*)")
+  end
+  return "en", "US", "UTF-8"
 end
 
 -- Checks if string is empty
@@ -6314,22 +6318,47 @@ function string:isempty()
   return self == ''
 end
 
+-- Changes newlines so everything is in one line
+-- @return string: String with formatted newlines as literal "\n"
+function string:linearize()
+  return self:gsub("\n", "\\n")
+end
+
+function string:indent(num, char)
+  num = num or 2
+  char = char or " "
+  char = string.rep(char, num)
+  return char .. self:gsub("\n", "\n" .. char)
+end
+
+-- Removes spaces at the beginning of the string
+-- @return string: left stripped string
 function string:lstrip()
   return self:gsub("^%s+", "")
 end
 
+-- Removes spaces at the end of the string
+-- @return string: right stripped string
 function string:rstrip()
   return self:gsub("%s+$", "")
 end
 
+-- Removes spaces at the beginning and at the end of the string
+-- @return string: stripped string
 function string:strip()
-  return self:lstrip():rstrip()
+  self = self:lstrip():rstrip()
+  return self
+end
+
+-- Alias of strip
+function string:trim()
+  return self:strip()
 end
 
 -- The following are heavily influenced by Python pathlib
 -- Check: https://docs.python.org/3/library/pathlib.html
 
--- Checks if string is a file or directory
+-- Checks if string is a file or a directory
 -- @return boolean: Exists or not
 function string:exists()
   return os.rename(self, self) ~= nil
@@ -6363,6 +6392,8 @@ function string:read_text()
   end
 end
 
+-- Gets file without suffix
+-- @return string: File wihtout suffix
 function string:stem()
   return self:gsub("%.%a+$", "")
 end
@@ -6394,6 +6425,8 @@ if pandoc ~= nil then
 
   -- Gets file extension namespace
   -- Check: https://pandoc.org/MANUAL.html#general-options
+  -- If suffix is 'md' or empty, the default is markdown.
+  -- For the rest, the format is the suffix.
   -- @param file string: File name
   -- @return string: File extension according to Pandoc format namespaces
   function pandoc.getext(file)
@@ -6403,22 +6436,21 @@ if pandoc ~= nil then
 
   -- Pandoc converter
   -- Converts input file name into output format name.
-  -- If ofile is not nil, writes output file name instead of returning content.
+  -- If ofile is not nil, writes output file name.
   -- If iformat is nil, defaults to ifile extension name.
   -- @param ifile string: Input file name
   -- @param oformat string: Output format name
   -- @param ofile string or nil: Output file name
   -- @param iformat string or nil: Input format name
-  -- @return string or nil: Output file content
+  -- @return string: Output file content
   function pandoc.convert(ifile, oformat, ofile, iformat)
     iformat = (iformat == nil and pandoc.getext(ifile) or iformat)
     local doc = pandoc.write(pandoc.read(ifile:read_text(), iformat), oformat)
-    if ofile == nil then
-      return doc
-    else
+    if ofile ~= nil then
       local eol = (os:isunix() and "\n" or "\r\n")
       io.open(ofile, "w"):write(doc, eol):close()
     end
+    return doc
   end
 
 end
@@ -6459,6 +6491,94 @@ lit.metastruct = {
   },
 }
 
+-- Messages for the user
+lit.msg = {
+  ["INFO"] = {
+    ["parsing"] = {
+      ["en"] = "Parsing:\n#1",
+      ["es"] = "Realizando análisis sintáctico:\n#1",
+    },
+  },
+  ["WARNING"] = {
+
+  },
+  ["ERROR"] = {
+    ["invalid_key"] = {
+      ["en"] = "Invalid key '#1' with value '#2'",
+      ["es"] = "Clave '#1' inválida con valor '#2'",
+    },
+    ["invalid_path"] = {
+      ["en"] = "Invalid path '#1' in key '#2'",
+      ["es"] = "Ruta '#1' inválida en clave '#2'",
+    },
+    ["invalid_type"] = {
+      ["en"] = "Invalid type '#1' in key '#2'",
+      ["es"] = "Tipo '#1' inválido en clave '#2'",
+    },
+    ["invalid_value"] = {
+      ["en"] = "Invalid value '#1' in key '#2'",
+      ["es"] = "Valor '#1' inválido en clave '#2'",
+    },
+    ["no_key"] = {
+      ["en"] = "Key '#1' not found",
+      ["es"] = "Clave '#1' no encontrada",
+    },
+    ["aborted"] = {
+      ["en"] = "Aborted due previous errors",
+      ["es"] = "Abortado debido a previos errores",
+    },
+    ["yaml_empty"] = {
+      ["en"] = "Empty YAML",
+      ["es"] = "YAML vacío",
+    },
+    ["yaml_invalid"] = {
+      ["en"] = "Invalid YAML",
+      ["es"] = "YAML inválido",
+    },
+  },
+}
+
+function lit.debuglevel(str)
+  if str == "ERROR" then
+    lit.status = false
+    return 2
+  elseif str == "WARNING" then
+    return 1
+  else
+    return 0
+  end
+end
+
+function lit.setmsg(msg, ...)
+  local lang = os.lang()
+  msg = (msg[lang] ~= nil and msg[lang] or msg) 
+  for i, str in ipairs({...}) do
+    msg = msg:gsub("#" .. i, str)
+  end
+  return msg
+end
+
+function lit.getmsg(key, ...)
+  local levelname = "INFO"
+  local level = 0
+  for mtype, msgs in pairs(lit.msg) do
+    if msgs[key] ~= nil then
+      key = lit.setmsg(msgs[key], ...)
+      levelname, level = mtype, lit.debuglevel(mtype)
+      break
+    end
+  end
+  return key, levelname, level 
+end
+
+function lit.puts(key, ...)
+  local verbosity = lit.debuglevel(PANDOC_STATE.verbosity)
+  local msg, levelname, level = lit.getmsg(key, ...)
+  if level >= verbosity then
+    print("[" .. levelname .. "] [LIT] " .. msg)
+  end
+end
+
 function lit.getmetaval(meta, key)
   local mtype = pandoc.utils.type(meta[key])
   local mval = meta[key]
@@ -6469,40 +6589,44 @@ function lit.getmetaval(meta, key)
   return mval, mtype
 end
 
+function lit.checkmetaextra(meta)
+  for key, val in pairs(meta) do
+    local missing1 = lit.metastruct["mandatory"][key] == nil
+    local missing2 = lit.metastruct["optional"][key] == nil
+    local mval = lit.getmetaval(meta, key) 
+    if missing1 and missing2 then
+      lit.puts("invalid_key", key, mval)
+    end
+  end
+end
+
 function lit.checkmetatype(type, meta, key)
   local mval, mtype = lit.getmetaval(meta, key)
-  local err = ""
   if type ~= mtype then
     if type == "path" then
       if not(pandoc.path.directory(mval):isdir()) then
-        err = "Invalid path '" .. mval .. "' in key '" .. key .. "'"
+        lit.puts("invalid_path", mval, key)
       end
     else
-      err = "Invalid type '" .. mtype .. "' in key '" .. key .. "'"
+      lit.puts("invalid_type", mtype, key)
     end
   end
-  if not(err:isempty()) then lit.puts(err, "ERROR") end
 end
 
 function lit.checkmetatable(table, meta, key)
   local mval = lit.getmetaval(meta, key) 
-  local err = ""
   for _, pattern in pairs(table) do
-    if mval:match("^" .. pattern .. "$") then
-      err = ""
-      break
-    else
-      err = "Invalid value '" .. mval .. "' in key '" .. key .. "'"
+    if mval:match("^" .. pattern .. "$")  == nil then
+      lit.puts("invalid_value", mval, key)
     end
   end
-  if not(err:isempty()) then lit.puts(err, "ERROR") end
 end
 
 function lit.checkmeta(meta, kind)
   for key, val in pairs(lit.metastruct[kind]) do
     if meta[key] == nil then
       if kind == "mandatory" then
-        lit.puts("Key '" .. key .. "' not found", "ERROR")
+        lit.puts("no_key", key)
       end
     else
       if type(val) == "table" then
@@ -6517,7 +6641,8 @@ end
 function lit.setmeta(meta)
   lit.checkmeta(meta, "mandatory")
   lit.checkmeta(meta, "optional")
-  -- TODO checks for 1) extra keys and 2) duplicates
+  lit.checkmetaextra(meta)
+  -- TODO checks for 2) duplicates
   -- Set defaults if lit.status = true
   return meta
 end
@@ -6562,51 +6687,32 @@ function lit.eval(code)
 end
 ]]--
 
+-- TODO
 function lit.assert(e, status)
   if status == false then
-    lit.puts("Aborted due previous errors", "ERROR")
+    lit.puts("aborted")
     os.exit(1)
   end
   return e
 end
 
-function lit.debuglevel(str)
-  if str == "ERROR" then
-    lit.status = false
-    return 2
-  elseif str == "WARNING" then
-    return 1
-  else
-    return 0
-  end
-end
-
-function lit.puts(msg, kind)
-  local kind = kind or "INFO"
-  local verbosity = lit.debuglevel(PANDOC_STATE.verbosity)
-  level = lit.debuglevel(kind)
-  if level >= verbosity then
-    print("[" .. kind .. "] [LIT] " .. msg)
-  end
-end
-
 function lit.parseyaml(rawyaml)
   local yaml = io.popen("pandoc -t json <<<\"" .. rawyaml .. "\" 2>&1")
   if yaml:read("*l"):sub(1, 1) == "{" then
     yaml = pandoc.read(rawyaml).meta
     if pandoc.utils.stringify(yaml):isempty() then
-      lit.puts("Empty YAML", "ERROR")
+      lit.puts("yaml_empty")
     else
       return lit.setmeta(yaml)
     end
   else
-    lit.puts("Invalid YAML", "ERROR")
+    lit.puts("yaml_invalid")
     return nil
   end
 end
 
 function lit.parseblock(parsed)
-  lit.puts("Parsing " .. table.concat(parsed, "\n"):linearize())
+  lit.puts("parsing", table.concat(parsed, ""):indent())
   local yaml = lit.parseyaml(parsed[1])
   -- TODO: parsecode
   return "TODO"
diff --git a/src/extensions.lua b/src/extensions.lua
index 88ae7b1..b8eef20 100644
--- a/src/extensions.lua
+++ b/src/extensions.lua
@@ -41,10 +41,14 @@ function os.isunix()
   return os.uname() ~= "windows"
 end
 
--- Changes newlines so everything is in one line
--- @return string: String with formatted newlines as literal "\n"
-function string:linearize()
-  return self:gsub("\n", "\\\\n")
+-- Gets OS language
+-- @return string, string, string: Language, locale and encoding
+function os.lang()
+  local lang = os.getenv("LANG")
+  if lang ~= nil then
+    return lang:match("(%w%w)_?(%w?%w?)%.?(.*)")
+  end
+  return "en", "US", "UTF-8"
 end
 
 -- Checks if string is empty
@@ -53,22 +57,47 @@ function string:isempty()
   return self == ''
 end
 
+-- Changes newlines so everything is in one line
+-- @return string: String with formatted newlines as literal "\n"
+function string:linearize()
+  return self:gsub("\n", "\\n")
+end
+
+function string:indent(num, char)
+  num = num or 2
+  char = char or " "
+  char = string.rep(char, num)
+  return char .. self:gsub("\n", "\n" .. char)
+end
+
+-- Removes spaces at the beginning of the string
+-- @return string: left stripped string
 function string:lstrip()
   return self:gsub("^%s+", "")
 end
 
+-- Removes spaces at the end of the string
+-- @return string: right stripped string
 function string:rstrip()
   return self:gsub("%s+$", "")
 end
 
+-- Removes spaces at the beginning and at the end of the string
+-- @return string: stripped string
 function string:strip()
-  return self:lstrip():rstrip()
+  self = self:lstrip():rstrip()
+  return self
+end
+
+-- Alias of strip
+function string:trim()
+  return self:strip()
 end
 
 -- The following are heavily influenced by Python pathlib
 -- Check: https://docs.python.org/3/library/pathlib.html
 
--- Checks if string is a file or directory
+-- Checks if string is a file or a directory
 -- @return boolean: Exists or not
 function string:exists()
   return os.rename(self, self) ~= nil
@@ -102,6 +131,8 @@ function string:read_text()
   end
 end
 
+-- Gets file without suffix
+-- @return string: File wihtout suffix
 function string:stem()
   return self:gsub("%.%a+$", "")
 end
@@ -133,6 +164,8 @@ if pandoc ~= nil then
 
   -- Gets file extension namespace
   -- Check: https://pandoc.org/MANUAL.html#general-options
+  -- If suffix is 'md' or empty, the default is markdown.
+  -- For the rest, the format is the suffix.
   -- @param file string: File name
   -- @return string: File extension according to Pandoc format namespaces
   function pandoc.getext(file)
@@ -142,22 +175,21 @@ if pandoc ~= nil then
 
   -- Pandoc converter
   -- Converts input file name into output format name.
-  -- If ofile is not nil, writes output file name instead of returning content.
+  -- If ofile is not nil, writes output file name.
   -- If iformat is nil, defaults to ifile extension name.
   -- @param ifile string: Input file name
   -- @param oformat string: Output format name
   -- @param ofile string or nil: Output file name
   -- @param iformat string or nil: Input format name
-  -- @return string or nil: Output file content
+  -- @return string: Output file content
   function pandoc.convert(ifile, oformat, ofile, iformat)
     iformat = (iformat == nil and pandoc.getext(ifile) or iformat)
     local doc = pandoc.write(pandoc.read(ifile:read_text(), iformat), oformat)
-    if ofile == nil then
-      return doc
-    else
+    if ofile ~= nil then
       local eol = (os:isunix() and "\n" or "\r\n")
       io.open(ofile, "w"):write(doc, eol):close()
     end
+    return doc
   end
 
 end
diff --git a/src/literate.lua b/src/literate.lua
index a70cd94..0345184 100644
--- a/src/literate.lua
+++ b/src/literate.lua
@@ -28,6 +28,94 @@ lit.metastruct = {
   },
 }
 
+-- Messages for the user
+lit.msg = {
+  ["INFO"] = {
+    ["parsing"] = {
+      ["en"] = "Parsing:\n#1",
+      ["es"] = "Realizando análisis sintáctico:\n#1",
+    },
+  },
+  ["WARNING"] = {
+
+  },
+  ["ERROR"] = {
+    ["invalid_key"] = {
+      ["en"] = "Invalid key '#1' with value '#2'",
+      ["es"] = "Clave '#1' inválida con valor '#2'",
+    },
+    ["invalid_path"] = {
+      ["en"] = "Invalid path '#1' in key '#2'",
+      ["es"] = "Ruta '#1' inválida en clave '#2'",
+    },
+    ["invalid_type"] = {
+      ["en"] = "Invalid type '#1' in key '#2'",
+      ["es"] = "Tipo '#1' inválido en clave '#2'",
+    },
+    ["invalid_value"] = {
+      ["en"] = "Invalid value '#1' in key '#2'",
+      ["es"] = "Valor '#1' inválido en clave '#2'",
+    },
+    ["no_key"] = {
+      ["en"] = "Key '#1' not found",
+      ["es"] = "Clave '#1' no encontrada",
+    },
+    ["aborted"] = {
+      ["en"] = "Aborted due previous errors",
+      ["es"] = "Abortado debido a previos errores",
+    },
+    ["yaml_empty"] = {
+      ["en"] = "Empty YAML",
+      ["es"] = "YAML vacío",
+    },
+    ["yaml_invalid"] = {
+      ["en"] = "Invalid YAML",
+      ["es"] = "YAML inválido",
+    },
+  },
+}
+
+function lit.debuglevel(str)
+  if str == "ERROR" then
+    lit.status = false
+    return 2
+  elseif str == "WARNING" then
+    return 1
+  else
+    return 0
+  end
+end
+
+function lit.setmsg(msg, ...)
+  local lang = os.lang()
+  msg = (msg[lang] ~= nil and msg[lang] or msg) 
+  for i, str in ipairs({...}) do
+    msg = msg:gsub("#" .. i, str)
+  end
+  return msg
+end
+
+function lit.getmsg(key, ...)
+  local levelname = "INFO"
+  local level = 0
+  for mtype, msgs in pairs(lit.msg) do
+    if msgs[key] ~= nil then
+      key = lit.setmsg(msgs[key], ...)
+      levelname, level = mtype, lit.debuglevel(mtype)
+      break
+    end
+  end
+  return key, levelname, level 
+end
+
+function lit.puts(key, ...)
+  local verbosity = lit.debuglevel(PANDOC_STATE.verbosity)
+  local msg, levelname, level = lit.getmsg(key, ...)
+  if level >= verbosity then
+    print("[" .. levelname .. "] [LIT] " .. msg)
+  end
+end
+
 function lit.getmetaval(meta, key)
   local mtype = pandoc.utils.type(meta[key])
   local mval = meta[key]
@@ -38,40 +126,44 @@ function lit.getmetaval(meta, key)
   return mval, mtype
 end
 
+function lit.checkmetaextra(meta)
+  for key, val in pairs(meta) do
+    local missing1 = lit.metastruct["mandatory"][key] == nil
+    local missing2 = lit.metastruct["optional"][key] == nil
+    local mval = lit.getmetaval(meta, key) 
+    if missing1 and missing2 then
+      lit.puts("invalid_key", key, mval)
+    end
+  end
+end
+
 function lit.checkmetatype(type, meta, key)
   local mval, mtype = lit.getmetaval(meta, key)
-  local err = ""
   if type ~= mtype then
     if type == "path" then
       if not(pandoc.path.directory(mval):isdir()) then
-        err = "Invalid path '" .. mval .. "' in key '" .. key .. "'"
+        lit.puts("invalid_path", mval, key)
       end
     else
-      err = "Invalid type '" .. mtype .. "' in key '" .. key .. "'"
+      lit.puts("invalid_type", mtype, key)
     end
   end
-  if not(err:isempty()) then lit.puts(err, "ERROR") end
 end
 
 function lit.checkmetatable(table, meta, key)
   local mval = lit.getmetaval(meta, key) 
-  local err = ""
   for _, pattern in pairs(table) do
-    if mval:match("^" .. pattern .. "$") then
-      err = ""
-      break
-    else
-      err = "Invalid value '" .. mval .. "' in key '" .. key .. "'"
+    if mval:match("^" .. pattern .. "$")  == nil then
+      lit.puts("invalid_value", mval, key)
     end
   end
-  if not(err:isempty()) then lit.puts(err, "ERROR") end
 end
 
 function lit.checkmeta(meta, kind)
   for key, val in pairs(lit.metastruct[kind]) do
     if meta[key] == nil then
       if kind == "mandatory" then
-        lit.puts("Key '" .. key .. "' not found", "ERROR")
+        lit.puts("no_key", key)
       end
     else
       if type(val) == "table" then
@@ -86,7 +178,8 @@ end
 function lit.setmeta(meta)
   lit.checkmeta(meta, "mandatory")
   lit.checkmeta(meta, "optional")
-  -- TODO checks for 1) extra keys and 2) duplicates
+  lit.checkmetaextra(meta)
+  -- TODO checks for 2) duplicates
   -- Set defaults if lit.status = true
   return meta
 end
@@ -131,51 +224,32 @@ function lit.eval(code)
 end
 ]]--
 
+-- TODO
 function lit.assert(e, status)
   if status == false then
-    lit.puts("Aborted due previous errors", "ERROR")
+    lit.puts("aborted")
     os.exit(1)
   end
   return e
 end
 
-function lit.debuglevel(str)
-  if str == "ERROR" then
-    lit.status = false
-    return 2
-  elseif str == "WARNING" then
-    return 1
-  else
-    return 0
-  end
-end
-
-function lit.puts(msg, kind)
-  local kind = kind or "INFO"
-  local verbosity = lit.debuglevel(PANDOC_STATE.verbosity)
-  level = lit.debuglevel(kind)
-  if level >= verbosity then
-    print("[" .. kind .. "] [LIT] " .. msg)
-  end
-end
-
 function lit.parseyaml(rawyaml)
   local yaml = io.popen("pandoc -t json <<<\"" .. rawyaml .. "\" 2>&1")
   if yaml:read("*l"):sub(1, 1) == "{" then
     yaml = pandoc.read(rawyaml).meta
     if pandoc.utils.stringify(yaml):isempty() then
-      lit.puts("Empty YAML", "ERROR")
+      lit.puts("yaml_empty")
     else
       return lit.setmeta(yaml)
     end
   else
-    lit.puts("Invalid YAML", "ERROR")
+    lit.puts("yaml_invalid")
     return nil
   end
 end
 
 function lit.parseblock(parsed)
-  lit.puts("Parsing " .. table.concat(parsed, "\n"):linearize())
+  lit.puts("parsing", table.concat(parsed, ""):indent())
   local yaml = lit.parseyaml(parsed[1])
   -- TODO: parsecode
   return "TODO"