gistfile1.rb

def matching(prop)
  @matching ||= inject({}) do |hash, p|
    (hash[p.root] ||= []) << p
    hash
  end[prop.root] || []
end

def matching(prop)
  @matching ||= begin
    map = {}
    each do |p|
      map[p.root] ||= []
      map[p.root] << p
    end
    map
  end
  @matching[prop.root] || []
end