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: shell.rb
    path of file: ./repos/ruby.fundamental/algorithms/sort
    
    the code of the file until where you have to start completion: # coding: utf-8
# Shell sort: Similar approach as insertion sort but slightly better.
# Requirements: Needs to be able to compare elements with <=>, and the []
    
  def shell_sort!
    inc = length / 2

    while inc != 0
      inc.step(length - 1) do |i|
        el = self[i]
        while i >= inc && self[i - inc] > el
          self[i] = self[i - inc]
          i -= inc
        end
        self[i] = el
      end