Set::SubclassCompatible#proper_superset?

# File tmp/rubies/ruby-4.0.0/lib/set/subclass_compatible.rb, line 125
def proper_superset?(set)
  case
  when set.instance_of?(self.class)
    super
  when set.is_a?(Set)
    size > set.size && set.all?(self)
  else
    raise ArgumentError, "value must be a set"
  end
end