diff --git a/exe/referator b/exe/referator
new file mode 100755
index 0000000..23f5df6
--- /dev/null
+++ b/exe/referator
@@ -0,0 +1,63 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+lib = File.expand_path('../lib', __dir__).freeze
+$LOAD_PATH.unshift lib unless $LOAD_PATH.include? lib
+
+require 'json'
+require 'referator'
+
+$stdin.sync = true
+$stdout.sync = true
+
+def parse_command(line)
+  line = String(line).chomp
+  command = line[0...line.index(' ')].freeze
+  rest = line[command.length..].strip.freeze
+  [command, rest].freeze
+end
+
+workdir = ARGV.first
+workdir = Dir.pwd if String(workdir).strip.empty?
+workdir = File.expand_path(workdir).freeze
+
+config = Referator::Config.new workdir
+footnotes = Referator::Footnotes.new config
+
+while (line = $stdin.gets)
+  command, rest = parse_command line
+  case command
+  when 'REGISTER_FORMAT'
+    data = JSON.parse rest
+    config.formats.register(
+      String(data['name']).to_sym,
+      **data.except('name').transform_keys(&:to_sym),
+    )
+  when 'REGISTER_KIND'
+    data = JSON.parse rest
+    config.kinds.register String(data).to_sym
+  when 'ADD_REF'
+    data = JSON.parse rest
+    config.repo.add_ref(**data.transform_keys(&:to_sym))
+  when 'ADD_NOTE'
+    config.freeze
+    data = JSON.parse rest
+    puts footnotes.add_note(
+      String(data['kind']).to_sym,
+      String(data['id']),
+    ).to_h.to_json
+  when 'REF'
+    config.freeze
+    data = JSON.parse rest
+    puts config.repo[
+      String(data['kind']).to_sym,
+      String(data['id']),
+    ].to_h.to_json
+  when 'RENDER_FOOTNOTES'
+    config.freeze
+    data = JSON.parse rest
+    puts footnotes.render(String(data).to_sym).to_json
+  else
+    raise 'Invalid command'
+  end
+end
diff --git a/lib/referator.rb b/lib/referator.rb
new file mode 100644
index 0000000..7016dc6
--- /dev/null
+++ b/lib/referator.rb
@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+require 'forwardable'
+require 'json'
+require 'open3'
+require 'pathname'
+
+require_relative 'referator/config'
+require_relative 'referator/config/formats'
+require_relative 'referator/config/kinds'
+require_relative 'referator/config/repo'
+require_relative 'referator/footnotes'
+require_relative 'referator/note'
+require_relative 'referator/reference'
+require_relative 'referator/script'
+
+module Referator
+  NAME_RE = /\A\w+(_\w+)*\z/
+  SLUG_RE = /\A\w+(-\w+)*\z/
+end
diff --git a/lib/referator/config.rb b/lib/referator/config.rb
new file mode 100644
index 0000000..8766494
--- /dev/null
+++ b/lib/referator/config.rb
@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+module Referator
+  class Config
+    attr_reader :workdir
+
+    def initialize(workdir)
+      self.workdir = workdir
+    end
+
+    def freeze
+      formats.freeze
+      kinds.freeze
+      repo.freeze
+      super
+    end
+
+    def formats(&)
+      @formats ||= Formats.new self
+    end
+
+    def kinds(&)
+      @kinds ||= Kinds.new self
+    end
+
+    def repo(&)
+      @repo ||= Repo.new self
+    end
+
+  private
+
+    def workdir=(workdir)
+      workdir = Pathname.new(workdir).expand_path.freeze
+      raise 'Expected absolute path' unless workdir.absolute?
+      raise 'Expected existing directory' unless workdir.directory?
+
+      @workdir = workdir
+    end
+  end
+end
diff --git a/lib/referator/config/formats.rb b/lib/referator/config/formats.rb
new file mode 100644
index 0000000..09efde1
--- /dev/null
+++ b/lib/referator/config/formats.rb
@@ -0,0 +1,62 @@
+# frozen_string_literal: true
+
+module Referator
+  class Config
+    class Formats
+      attr_reader :config, :names, :scripts
+
+      def initialize(config)
+        self.config = config
+        @names = []
+        @scripts = {}
+      end
+
+      def freeze
+        @names.freeze
+        @scripts.freeze
+        super
+      end
+
+      def register(name, **kwargs)
+        validate_name! name
+        raise 'Already exists' if names.index(name) || scripts.key?(name)
+
+        script = Script.new(**kwargs.merge(workdir: config.workdir,
+                                           vars: script_vars.keys))
+
+        names << name
+        scripts[name] = script
+        nil
+      end
+
+      def exists!(name)
+        scripts[name] or raise 'Unknown format'
+        nil
+      end
+
+      def render(name, notes)
+        exists! name
+        scripts[name].call(**script_vars(format: name, notes: notes.to_json))
+      end
+
+    private
+
+      def config=(config)
+        unless config.instance_of? Config
+          raise TypeError, "Expected #{Config}, got #{config.class}"
+        end
+
+        @config = config
+      end
+
+      def validate_name!(name)
+        unless name.instance_of? Symbol
+          raise TypeError, "Expected #{Symbol}, got #{name.class}"
+        end
+        raise 'Invalid name' unless NAME_RE.match? name
+      end
+
+      def script_vars(format: nil, notes: nil) = { format:, notes: }.freeze
+    end
+  end
+end
diff --git a/lib/referator/config/kinds.rb b/lib/referator/config/kinds.rb
new file mode 100644
index 0000000..f403153
--- /dev/null
+++ b/lib/referator/config/kinds.rb
@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+module Referator
+  class Config
+    class Kinds
+      attr_reader :config, :names
+
+      def initialize(config)
+        self.config = config
+        @names = []
+      end
+
+      def freeze
+        @names.freeze
+        super
+      end
+
+      def register(name)
+        validate_name! name
+        raise 'Already exists' if names.index(name)
+
+        names << name
+        nil
+      end
+
+      def exists!(name)
+        names.index name or raise 'Unknown kind'
+        nil
+      end
+
+    private
+
+      def config=(config)
+        unless config.instance_of? Config
+          raise TypeError, "Expected #{Config}, got #{config.class}"
+        end
+
+        @config = config
+      end
+
+      def validate_name!(name)
+        unless name.instance_of? Symbol
+          raise TypeError, "Expected #{Symbol}, got #{name.class}"
+        end
+        raise 'Invalid name' unless NAME_RE.match? name
+      end
+    end
+  end
+end
diff --git a/lib/referator/config/repo.rb b/lib/referator/config/repo.rb
new file mode 100644
index 0000000..3c0ce88
--- /dev/null
+++ b/lib/referator/config/repo.rb
@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+module Referator
+  class Config
+    class Repo
+      attr_reader :config
+
+      def initialize(config)
+        self.config = config
+        @references = {}
+      end
+
+      def freeze
+        @references.freeze
+        super
+      end
+
+      def add_ref(**kwargs)
+        reference = Reference.new(**kwargs)
+        key = "#{reference.kind}-#{reference.id}".freeze
+        raise 'Reference already exists' if @references.key? key
+
+        @references[key] = reference
+        nil
+      end
+
+      def [](kind, id)
+        config.kinds.exists! kind
+        key = "#{kind}-#{id}".freeze
+        @references[key].tap do |reference|
+          raise 'Invalid reference' if reference.nil?
+        end
+      end
+
+    private
+
+      def config=(config)
+        unless config.instance_of? Config
+          raise TypeError, "Expected #{Config}, got #{config.class}"
+        end
+
+        @config = config
+      end
+    end
+  end
+end
diff --git a/lib/referator/footnotes.rb b/lib/referator/footnotes.rb
new file mode 100644
index 0000000..03b0e0c
--- /dev/null
+++ b/lib/referator/footnotes.rb
@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+module Referator
+  class Footnotes
+    attr_reader :config
+
+    def initialize(config)
+      self.config = config
+
+      @notes = []
+    end
+
+    def add_note(kind, id)
+      config.kinds.exists! kind
+      reference = config.repo[kind, id]
+      new_note = Note.new self, @notes.count + 1, reference
+      @notes.each { |note| return note if note.eql? new_note }
+      @notes << new_note
+      new_note
+    end
+
+    def render(format)
+      config.formats.render format, @notes.map(&:to_h).freeze
+    end
+
+  private
+
+    def config=(config)
+      unless config.instance_of? Config
+        raise TypeError, "Expected #{Config}, got #{config.class}"
+      end
+
+      @config = config
+    end
+  end
+end
diff --git a/lib/referator/note.rb b/lib/referator/note.rb
new file mode 100644
index 0000000..a95afec
--- /dev/null
+++ b/lib/referator/note.rb
@@ -0,0 +1,54 @@
+# frozen_string_literal: true
+
+module Referator
+  class Note
+    extend Forwardable
+
+    attr_reader :footnotes, :index, :reference
+
+    def_delegators :reference, :anchor, :fragment
+
+    def initialize(footnotes, index, reference)
+      self.footnotes = footnotes
+      self.index = index
+      self.reference = reference
+    end
+
+    def to_h
+      @to_h ||= reference.to_h.merge(index:).freeze
+    end
+
+    def eql?(other)
+      other.instance_of?(self.class) &&
+        footnotes == other.footnotes &&
+        anchor == other.anchor
+    end
+
+    def ==(other) = eql?(other) && index == other.index
+
+  private
+
+    def footnotes=(footnotes)
+      unless footnotes.instance_of? Footnotes
+        raise TypeError, "Expected #{Footnotes}, got #{footnotes.class}"
+      end
+
+      @footnotes = footnotes
+    end
+
+    def index=(index)
+      index = Integer index
+      raise ArgumentError, 'Invalid index' unless index.positive?
+
+      @index = index
+    end
+
+    def reference=(reference)
+      unless reference.is_a? Reference
+        raise TypeError, "Expected #{Reference}, got #{reference.class}"
+      end
+
+      @reference = reference
+    end
+  end
+end
diff --git a/lib/referator/reference.rb b/lib/referator/reference.rb
new file mode 100644
index 0000000..78f3ffd
--- /dev/null
+++ b/lib/referator/reference.rb
@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+module Referator
+  class Reference
+    attr_reader :kind, :id, :slug, :data
+
+    def initialize(kind:, id:, slug:, **kwargs)
+      self.kind = kind
+      self.id   = id
+      self.slug = slug
+      self.data = kwargs
+    end
+
+    def to_h
+      @to_h ||= data.merge(kind:, id:, slug:, anchor:, fragment:).freeze
+    end
+
+    def anchor
+      @anchor ||= "#{kind}-#{slug}".freeze
+    end
+
+    def fragment
+      @fragment ||= "##{anchor}".freeze
+    end
+
+  private
+
+    def kind=(kind)
+      @kind = String(kind).to_sym.tap do |new_kind|
+        raise 'Invalid kind' unless NAME_RE.match? new_kind
+      end
+    end
+
+    def id=(id)
+      @id = String(id).freeze
+    end
+
+    def slug=(slug)
+      @slug = String(slug).freeze.tap do |new_slug|
+        raise 'Invalid slug' unless SLUG_RE.match? new_slug
+      end
+    end
+
+    def data=(data)
+      @data = Hash(data).transform_keys { |key| String(key).to_sym }.freeze
+    end
+  end
+end
diff --git a/lib/referator/script.rb b/lib/referator/script.rb
new file mode 100644
index 0000000..9e05096
--- /dev/null
+++ b/lib/referator/script.rb
@@ -0,0 +1,130 @@
+# frozen_string_literal: true
+
+module Referator
+  class Script
+    attr_reader :workdir, :vars, :args, :env, :stdin
+
+    def initialize(workdir:, vars:, args: [], env: {}, stdin: [])
+      self.workdir = workdir
+      self.vars = vars
+
+      self.args = args
+      self.env = env
+      self.stdin = stdin
+
+      raise 'Invalid script' if self.args.empty?
+    end
+
+    def call(**kwargs)
+      raise 'Invalid vars' unless kwargs.keys.sort == vars
+
+      stdout, status = Open3.capture2(
+        sub_env(**kwargs),
+        *sub_args(**kwargs),
+        chdir: workdir.to_s,
+        stdin_data: sub_stdin(**kwargs),
+      )
+      raise 'Exit code' unless status.success?
+
+      String(stdout).freeze
+    end
+
+  private
+
+    def workdir=(workdir)
+      workdir = Pathname.new(workdir).expand_path.freeze
+      raise 'Expected absolute path' unless workdir.absolute?
+      raise 'Expected existing directory' unless workdir.directory?
+
+      @workdir = workdir
+    end
+
+    def vars=(vars)
+      @vars = [*vars].uniq.sort.freeze.each do |var|
+        unless var.instance_of? Symbol
+          raise TypeError, "Expected #{Symbol}, got #{var.class}"
+        end
+      end
+    end
+
+    def args=(args)
+      @args = [*args].map { |arg| Sub.new vars, arg }.freeze
+    end
+
+    def env=(env)
+      @env = Hash(env).to_h do |key, value|
+        [String(key).freeze, Sub.new(vars, value)]
+      end.freeze
+    end
+
+    def stdin=(stdin)
+      @stdin = Sub.new vars, stdin
+    end
+
+    def sub_args(**kwargs)
+      args.map { |sub| sub.call(**kwargs) }.freeze
+    end
+
+    def sub_env(**kwargs)
+      env.transform_values { |sub| sub.call(**kwargs) }.freeze
+    end
+
+    def sub_stdin(**kwargs)
+      stdin.call(**kwargs)
+    end
+
+    class Sub
+      attr_reader :vars, :template
+
+      def initialize(vars, template)
+        self.vars = vars
+
+        self.template =
+          if template.instance_of?(String) ||
+             template.instance_of?(Symbol) ||
+             template.instance_of?(Hash)
+            [template]
+          else
+            template
+          end
+      end
+
+      def call(**kwargs)
+        raise 'Invalid vars' unless kwargs.keys.sort == vars
+
+        template.map do |part|
+          if part.instance_of? String
+            part
+          else
+            kwargs.fetch part
+          end
+        end.join.freeze
+      end
+
+    private
+
+      def vars=(vars)
+        @vars = [*vars].uniq.sort.freeze.each do |var|
+          unless var.instance_of? Symbol
+            raise TypeError, "Expected #{Symbol}, got #{var.class}"
+          end
+        end
+      end
+
+      def template=(template)
+        raise 'Invalid format' unless template.instance_of? Array
+
+        @template = template.map do |part|
+          part = String(part.keys.first).to_sym if part.instance_of? Hash
+
+          unless part.instance_of?(String) ||
+                 part.instance_of?(Symbol) && vars.include?(part)
+            raise 'Invalid format'
+          end
+
+          part.freeze
+        end.freeze
+      end
+    end
+  end
+end
diff --git a/spec/lib/referator/script/sub_spec.rb b/spec/lib/referator/script/sub_spec.rb
new file mode 100644
index 0000000..637f813
--- /dev/null
+++ b/spec/lib/referator/script/sub_spec.rb
@@ -0,0 +1,72 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Referator::Script::Sub do
+  subject(:sub) { described_class.new vars, template }
+
+  let(:vars) { %i[foo bar car].freeze }
+  let(:template) { ['qwe', :foo, 'rty', { bar: nil }.freeze].freeze }
+
+  describe '#call' do
+    subject(:result) { sub.call(**kwargs) }
+
+    let(:kwargs) { { foo: '123', bar: '456', car: '789' }.freeze }
+
+    it { is_expected.to be_instance_of String }
+    it { is_expected.to be_frozen }
+    it { is_expected.to eq 'qwe123rty456' }
+
+    context 'when the "vars" array is empty' do
+      let(:vars) { [].freeze }
+      let(:template) { %w[qwe rty].freeze }
+      let(:kwargs) { {}.freeze }
+
+      it { is_expected.to be_instance_of String }
+      it { is_expected.to be_frozen }
+      it { is_expected.to eq 'qwerty' }
+    end
+
+    context 'when template is empty' do
+      let(:template) { [].freeze }
+
+      it { is_expected.to be_instance_of String }
+      it { is_expected.to be_frozen }
+      it { is_expected.to be_empty }
+    end
+
+    context 'when the "vars" array and template are empty' do
+      let(:vars) { [].freeze }
+      let(:template) { [].freeze }
+      let(:kwargs) { {}.freeze }
+
+      it { is_expected.to be_instance_of String }
+      it { is_expected.to be_frozen }
+      it { is_expected.to be_empty }
+    end
+
+    context 'when template is a String' do
+      let(:template) { 'qwe' }
+
+      it { is_expected.to be_instance_of String }
+      it { is_expected.to be_frozen }
+      it { is_expected.to eq 'qwe' }
+    end
+
+    context 'when template is a Symbol' do
+      let(:template) { :foo }
+
+      it { is_expected.to be_instance_of String }
+      it { is_expected.to be_frozen }
+      it { is_expected.to eq '123' }
+    end
+
+    context 'when template is a Hash' do
+      let(:template) { { foo: nil }.freeze }
+
+      it { is_expected.to be_instance_of String }
+      it { is_expected.to be_frozen }
+      it { is_expected.to eq '123' }
+    end
+  end
+end
diff --git a/spec/lib/referator/script_spec.rb b/spec/lib/referator/script_spec.rb
new file mode 100644
index 0000000..71d00aa
--- /dev/null
+++ b/spec/lib/referator/script_spec.rb
@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Referator::Script do
+  subject(:script) { described_class.new workdir:, vars:, args:, env:, stdin: }
+
+  let(:workdir) { Pathname.new(__dir__).join('../../..').expand_path.freeze }
+  let(:vars) { %i[foo bar car].freeze }
+
+  let(:args) { ['echo', :foo, { bar: nil }, :car].freeze }
+  let(:env) { {}.freeze }
+  let(:stdin) { [].freeze }
+
+  describe '#call' do
+    subject(:result) { script.call(**kwargs) }
+
+    let(:kwargs) { { foo: '123', bar: '456', car: '789' }.freeze }
+
+    it { is_expected.to be_instance_of String }
+    it { is_expected.to be_frozen }
+    it { is_expected.to eq "123 456 789\n" }
+
+    context 'when it depends on env vars' do
+      let(:args) { ['sh', '-c', ['echo ', :foo, ' $BAR $CAR'].freeze].freeze }
+      let(:env) { { BAR: :bar, CAR: [:car].freeze }.freeze }
+
+      it { is_expected.to be_instance_of String }
+      it { is_expected.to be_frozen }
+      it { is_expected.to eq "123 456 789\n" }
+    end
+
+    context 'when it depends on stdin' do
+      let(:args) { 'cat' }
+      let(:stdin) { [:foo, "\n", :bar, "\n", :car, "\n"].freeze }
+
+      it { is_expected.to be_instance_of String }
+      it { is_expected.to be_frozen }
+      it { is_expected.to eq "123\n456\n789\n" }
+    end
+  end
+end