proper_subset?

# File lib/set.rb, line 277
  def proper_subset?(set)
    case
    when set.instance_of?(self.class)
      @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