function stringAdd(string _a,string _b) private returns(string){
        bytes memory a = bytes(_a);
        bytes memory b = bytes(_b);
        string memory _ab = new string(a.length + b.length);
        bytes memory ab = bytes(_ab);
        uint k = 0;
        for (uint i = 0; i < a.length; i++) ab[k++] = a[i];
        for (i = 0; i < b.length; i++) ab[k++] = b[i];
        return string(ab);

    }