# File lib/rvg/text.rb, line 156
        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