alphabet_testers.rb

module AlphabetTesters
  def self.a? letter
    letter.downcase == "a"
  end

  def self.b? letter
    letter.downcase == "b"
  end

  def a? letter
    letter.downcase == "a"
  end

  def b? letter
    letter.downcase == "b"
  end
end

test_alphabet.rb

require_relative "alphabet_testers"
 include AlphabetTesters

puts AlphabetTesters.a? "a"
puts AlphabetTesters.b? "b"
puts b? "b"
puts a? "b"