Trying all permutations of a word against a dictionary

anagrams.rb

def try_all_permutations(word, dictionary, pos)
  if word[pos].nil?
    raise "Position #{pos} not in word list"
  elsif pos == (word.length - 1)
    puts word if dictionary.member? word
  else
    try_all_permutations(word, dictionary, pos+1)

    for i in ((pos+1)..(word.length-1))
      word[i], word[pos] = word[pos], word[i]
      try_all_permutations(word, dictionary, pos+1)
      word[i], word[pos] = word[pos], word[i]
    end 
  end 
end

try_all_permutations "tac", %w{cat act}, 0