basic_string<>::_M_mutate

[Benjamin Kosnik]

This and the operator+ stuff in.

Thanks,
Benjamin

> 1999-07-12  Ryszard Kabatek <kabatek@chemie.uni-halle.de>
> 
>         * bits/string.tcc: _M_mutate - clear _M_state again.
> 	* bits/basic_string.h: In the three functions that return
> 	  an iterator set _M_state to -1 bevore return.