Set::SubclassCompatible#classify

# File tmp/rubies/ruby-4.0.0/lib/set/subclass_compatible.rb, line 300
def classify
  block_given? or return enum_for(__method__) { size }

  h = {}

  each { |i|
    (h[yield(i)] ||= self.class.new).add(i)
  }

  h
end