You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: lean.rb
    path of file: ./repos/rouge/lib/rouge/lexers
    
    the code of the file until where you have to start completion: # -*- coding: utf-8 -*- #
# frozen_string_literal: true

# NOTE: This is for Lean 3 (community fork).
module Rouge
  module Lexers
    class Lean < RegexLexer
      title 'Lean'
      desc 'The Lean programming language (leanprover.github.io)'
      tag 'lean'
      aliases 'lean'
      filenames '*.lean'

      def self.keywords
        @keywords ||= 
    
      def self.keywords
        @keywords ||= Set.new %w(
          abbreviation
          add_rewrite
          alias
          assume
          axiom
          begin
          by
          calc
          calc_refl
          calc_subst
          calc_trans
          #check
          coercion
          conjecture
          constant
          constants
          context
          corollary
          def
          definition
          end