diff --git a/.rubocop.yml b/.rubocop.yml
index 8f30860..c3f2229 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -49,3 +49,5 @@ Layout/EmptyLineAfterGuardClause:
   Enabled: false
 Layout/CaseIndentation:
   Enabled: false
+Naming/MethodParameterName:
+  MinNameLength: 2
diff --git a/lib/judges/fb/once.rb b/lib/judges/fb/once.rb
new file mode 100644
index 0000000..a780aa4
--- /dev/null
+++ b/lib/judges/fb/once.rb
@@ -0,0 +1,61 @@
+# frozen_string_literal: true
+
+# Copyright (c) 2024 Yegor Bugayenko
+#
+# 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.
+
+# Returns a decorated global factbase, which only touches facts once
+def once(fb)
+  Factbase::Once.new(fb, $judge)
+end
+
+# Runs only once.
+# Author:: Yegor Bugayenko (yegor256@gmail.com)
+# Copyright:: Copyright (c) 2024 Yegor Bugayenko
+# License:: MIT
+class Factbase::Once
+  def initialize(fb, func)
+    @fb = fb
+    @func = func
+  end
+
+  def query(expr)
+    expr = "(and #{expr} (not (eq seen '#{@func}')))"
+    After.new(@fb.query(expr), @func)
+  end
+
+  def insert
+    @fb.insert
+  end
+
+  # What happens after a fact is processed.
+  class After
+    def initialize(query, func)
+      @query = query
+      @func = func
+    end
+
+    def each
+      @query.each do |f|
+        yield f
+        f.seen = @func
+      end
+    end
+  end
+end
diff --git a/lib/judges/pack.rb b/lib/judges/pack.rb
index 8767506..e3f8d22 100644
--- a/lib/judges/pack.rb
+++ b/lib/judges/pack.rb
@@ -22,6 +22,7 @@
 
 require 'yaml'
 require_relative '../judges'
+require_relative '../judges/fb/once'
 
 # A single pack.
 # Author:: Yegor Bugayenko (yegor256@gmail.com)
@@ -37,6 +38,7 @@ def initialize(dir)
   # Run it with the given Factbase and environment variables.
   def run(fbase, env)
     $fb = fbase
+    $judge = File.basename(File.dirname(@dir))
     env.each do |k, v|
       # rubocop:disable Security/Eval
       eval("$#{k} = '#{v}'", binding, __FILE__, __LINE__) # $foo = 42
diff --git a/test/test_pack.rb b/test/test_pack.rb
index 9701480..e685386 100644
--- a/test/test_pack.rb
+++ b/test/test_pack.rb
@@ -53,4 +53,12 @@ def test_run_isolated
       assert_equal(1, fb2.size)
     end
   end
+
+  def test_with_supplemenary_functions
+    Dir.mktmpdir do |d|
+      File.write(File.join(d, 'x.rb'), 'once($fb).insert')
+      pack = Judges::Pack.new(d)
+      pack.run(Factbase.new, {})
+    end
+  end
 end