# File lib/rvg/text.rb, line 157
            def tref(obj, x=nil, y=nil)
                if ! obj.kind_of?(Tspan)
                    raise ArgumentError, "wrong argument type #{obj.class} (expected Tspan)"
                end
                obj = obj.deep_copy
                obj.parent = self
                tref = Tref.new(obj, x, y, self)
                @tspans << tref
                return tref
            end