diff --git a/luamml-pdf.sty b/luamml-pdf.sty
index 7f1d751..0e2419d 100644
--- a/luamml-pdf.sty
+++ b/luamml-pdf.sty
@@ -77,6 +77,12 @@
   \__luamml_pdf_showlists:
 }
 
+\NewDocumentCommand \RegisterFamilyMapping {m m} {
+  \iow_log:x {
+    LUAMML_INSTRUCTION:REGISTER_MAPPING: \int_use:N #1 : #2
+  }
+}
+
 \endinput
 
 \cs_new:Npn \__luamml_patch_package:nn #1 #2 {
diff --git a/pdfmml-logreader.lua b/pdfmml-logreader.lua
index 384345a..377f92d 100644
--- a/pdfmml-logreader.lua
+++ b/pdfmml-logreader.lua
@@ -14,11 +14,18 @@ local luamml_block = l.Cg('LUAMML_FORMULA_BEGIN:' * id * l.P'\n'^1 * l.Ct(
 
 local luamml_mark = l.Cg('LUAMML_MARK:' * id * ':' * l.Cs((1 - l.P'\n' + l.Cg('\n' * l.Cc'' - '\nLUAMML_MARK_END\n'))^0) * '\nLUAMML_MARK_END\n' * l.Cc'marks')
 
+local luamml_instruction = l.Cg('LUAMML_INSTRUCTION:' * l.Cc(nil) * l.C((1 - l.P'\n')^0) * '\n' * l.Cc'instructions')
+
 local function multi_table_set(t, key, value, table)
-  assert(t[table])[key] = value
+  table = t[table]
+  table[key or #table + 1] = value
   return t
 end
-local log_file = l.Cf(l.Ct(l.Cg(l.Ct'', 'groups') * l.Cg(l.Ct'', 'marks')) * (luamml_block + luamml_mark + line)^0, multi_table_set)
+local log_file = l.Cf(l.Ct(l.Cg(l.Ct'', 'groups')
+                    * l.Cg(l.Ct'', 'marks')
+                    * l.Cg(l.Ct'', 'instructions'))
+               * (luamml_block + luamml_mark + luamml_instruction + line)^0,
+               multi_table_set)
 
 return function(filename)
   local f
diff --git a/pdfmml.lua b/pdfmml.lua
index 58f509b..2b45dcb 100755
--- a/pdfmml.lua
+++ b/pdfmml.lua
@@ -4,10 +4,6 @@ local convert = require'luamml-convert'
 local mappings = require'luamml-legacy-mappings'
 local to_xml = require'luamml-xmlwriter'
 
-convert.register_family(1, mappings.oml)
-convert.register_family(2, mappings.oms)
-convert.register_family(3, mappings.omx)
-
 local parse_showlists = require'pdfmml-showlists'
 local parse_log = require'pdfmml-logreader'
 
@@ -30,9 +26,23 @@ if #arg < 1 then
     If {outname} includes {}, then a separate file is written for every formula with {} replaced by the formula id.\n', arg[0]))
   os.exit(1)
 end
-local math_lists = assert(parse_log(assert(try_extensions(arg[1], '.tml', '.log'),
+local parsed = assert(parse_log(assert(try_extensions(arg[1], '.tml', '.log'),
       "Couldn't find input file.")))
 
+for i, inst in ipairs(parsed.instructions) do
+  local _, _, family, mapping_name = inst:find'^REGISTER_MAPPING:([0-9]+):(.*)$'
+  if family then
+    local mapping = mappings[mapping_name]
+    if mapping then
+      convert.register_family(tonumber(family), mapping)
+    else
+      io.stderr:write(string.format('Unknown mapping %s ignored\n', mapping_name))
+    end
+  else
+    io.stderr:write'Unknown instruction ignored\n'
+  end
+end
+
 local out_prefix, out_suffix, out_stream
 if not arg[2] or arg[2] == '-' then
   out_stream = io.stdout
@@ -42,10 +52,10 @@ else
     out_stream = assert(io.open(arg[2], 'w'))
   end
 end
-for i, block in ipairs(math_lists.groups) do
+for i, block in ipairs(parsed.groups) do
   local stream = out_stream or assert(io.open(out_prefix .. tostring(i) .. out_suffix, 'w'))
   block = block[1]
-  local parsed = parse_showlists(block, nil, nil, math_lists.marks)
+  local parsed = parse_showlists(block, nil, nil, parsed.marks)
   local style = block.display and 0 or 2
   stream:write(
     to_xml(convert.make_root(convert.process(parsed, style), style)), '\n'
diff --git a/test_pdf.tex b/test_pdf.tex
index 5d14c49..9b459f7 100644
--- a/test_pdf.tex
+++ b/test_pdf.tex
@@ -1,9 +1,9 @@
 \documentclass{article}
 \usepackage{luamml-pdf}
 
-% \RegisterFamilyMapping\symsymbols{oms}
-% \RegisterFamilyMapping\symletters{oml}
-% \RegisterFamilyMapping\symlargesymbols{omx}
+\RegisterFamilyMapping\symsymbols{oms}
+\RegisterFamilyMapping\symletters{oml}
+\RegisterFamilyMapping\symlargesymbols{omx}
 
 \ExplSyntaxOn
 \cs_set_eq:NN \WriteoutFormula \luamml_pdf_write: