gistfile1.rb

keys = [
  %w[a 1 x],
  %w[a 1 y],
  %w[a 2 x],
  %w[a 3 x],
  %w[a 3 y],
  %w[b 3 y],
  %w[b 4 z],
]

last_key = keys.first.map { nil }
keys.each do |key|
  first_output_index = last_key.zip(key).each_with_index.detect do |pair, index|
    pair[0] != pair[1]
  end[1]
  output_key = ([nil] * first_output_index) + key[first_output_index..-1]
  last_key = key

  puts key.inspect + ' ' + output_key.inspect
end