def self.cache(name, &p)
  raise "no block given" unless block_given?
  n, v = *((p.arity<0)? [-p.arity-1,true] : [p.arity,false])
  args = (('a'..'z').to_a[0,n]+(v ? ['*args']:[])).join(',')
  eval "define_method(:_#{name}) {|#{args}| p.call(#{args})}"
  eval "define_method(:#{name})  {|#{args}| (@cache ||= {})[[name,#{args}]] ||= _#{name}(#{args})}"
  eval "define_method(:#{name}!) {|#{args}| (@cache ||= {})[[name,#{args}]]   = _#{name}(#{args})}"
end