class Set

def delete_if
  block_given? or return enum_for(__method__)
  to_a.each { |o| @hash.delete(o) if yield(o) }
  self
end unless method_defined? :delete_if

def keep_if
  block_given? or return enum_for(__method__)
  to_a.each { |o| @hash.delete(o) unless yield(o) }
  self
end unless method_defined? :keep_if