Problem with sorting algorithm in RailsWizard

sort_test.rb

require 'yaml'
require 'pp'
hash = YAML.load '
a:
  after: 
  before: 
l:
  after: 
  before: 
n:
  after: 
  before: l
b:
  after: 
  before: 
m:
  after: 
  before: 
c:
  after: 
  before: 
z:
  after: 
  before: 
'
hash.each do |k,v|
  v.each do |vk,vv|
    v[vk] = (vv||'').split
  end
end

class Hash
  def without *ks
    delete_if { |k,v| ks.include? k }
  end
  def delete! *ks
    #print "\n\nwithout: #{ks.join(', ')}"
    replace without *ks
  end
end

def print_keys hash
  puts hash.map(&:first).join('  ')
end

def sort hash
  sorted_array = hash.sort do |a,b|
    if b.last['after'].include?(a.first) || a.last['before'].include?(b.first)
     -1
    elsif b.last['before'].include?(a.first) || b.last['after'].include?(b.first)
      1
    else
      a.first <=> b.first # simple sort by keys
    end
  end
  print_keys sorted_array
  Hash[sorted_array]
end

def test hash
  puts nil,nil
  print_keys hash
  puts '-'*hash.count*3
  hash.count.times { hash = sort hash }
end

test hash
test hash.delete! 'z'
test hash.delete! 'c'
test hash.delete! 'b'
test hash.delete! 'a'
test hash.delete! 'm'

stdout

a  l  n  b  m  c  z
---------------------
a  b  c  m  n  l  z
a  b  c  l  m  n  z
a  b  c  n  l  m  z
a  b  c  m  n  l  z
a  b  c  l  m  n  z
a  b  c  n  l  m  z
a  b  c  m  n  l  z


a  l  n  b  m  c
------------------
a  b  c  l  m  n
a  b  c  m  n  l
a  b  c  n  l  m
a  b  c  l  m  n
a  b  c  m  n  l
a  b  c  n  l  m


a  l  n  b  m
---------------
a  b  l  m  n
a  b  m  n  l
a  b  n  l  m
a  b  l  m  n
a  b  m  n  l


a  l  n  m
------------
a  l  m  n
a  l  m  n
a  l  m  n
a  l  m  n


l  n  m
---------
m  n  l
m  n  l
m  n  l


l  n
------
n  l
n  l