Set::SubclassCompatible#merge

# File tmp/rubies/ruby-4.1.0-preview1/lib/set/subclass_compatible.rb, line 228
def merge(*enums, **nil)
  enums.each do |enum|
    if enum.instance_of?(self.class)
      super(enum)
    else
      do_with_enum(enum) { |o| add(o) }
    end
  end

  self
end