Set#<

# File tmp/rubies/ruby-3.1.3/lib/set.rb, line 448
def proper_subset?(set)
  case
  when set.instance_of?(self.class) && @hash.respond_to?(:<)
    @hash < set.instance_variable_get(:@hash)
  when set.is_a?(Set)
    size < set.size && all? { |o| set.include?(o) }
  else
    raise ArgumentError, "value must be a set"
  end
end