def +(other)
      if self.class == Double || [Double, ::Float].include?(other.class)
        RDF::Literal::Double.new(to_f + other.to_f)
      elsif ((self.class == RDF::Literal::Float || other.class == RDF::Literal::Float) rescue false)
        RDF::Literal::Float.new(to_f + other.to_f)