    # 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