for( i = 1,j=0; i <poz,j<strlen(s1.str); i++,j++)
        {buff.str[i] = s1.str[i];buff.output();}

     for(i=poz,j=1;i<=strlen(s2.str),j<=strlen(s2.str);i++,j++)
        {buff.str[i] = s2.str[j];buff.output();}

     for(i=strlen(s2.str)+poz,j=poz+1;i<strlen(s1.str),j<strlen(s1.str)+poz;i++,j++)
        {buff.str[i]=s1.str[j];buff.output();}