+If \arg{str} is used, data written to the stream is appended to the current
+contents of it, i.e. the string is not cleared here. However if it is not
+empty, the positions returned by \helpref{TellO}{wxoutputstreamtello} will be
+offset by the initial string length, i.e. initial stream position will be the
+initial length of the string and not $0$.
+