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: output.rb
    path of file: ./repos/bat/tests/syntax-tests/source/Ruby
    
    the code of the file until where you have to start completion: class RepeatedSubstring
  def find_repeated_substring(s)
    # catch the edge cases
    return 'NONE' if s == ''
    # check if the string consists of only one character => "aaaaaa" => "a"
    ret
    
  def get_prefix(s1, s2)
    prefix = ''
    min_length = [s1.length, s2.length].min
    return '' if s1.nil? || s2.nil?

    (0..min_length - 1).each do |i|
      return prefix if s1[i] != s2[i]

      prefix += s1[i]
    end
    prefix
  end