Set::SubclassCompatible#<

# File tmp/rubies/ruby-4.0.0/lib/set/subclass_compatible.rb, line 149
def proper_subset?(set)
  case
  when set.instance_of?(self.class)
    super
  when set.is_a?(Set)
    size < set.size && all?(set)
  else
    raise ArgumentError, "value must be a set"
  end
end