Stringa operator -(Stringa s1, Stringa s2){
    int l;
    QString r;
    if(s1.length() > s2.length())
        l = s1.length();
    else
        l = s2.length();
    for(int i = 0; i < l; i++){
        char c1 = s1.charAt(i).toLatin1();
        char c2 = s2.charAt(i).toLatin1();
        int ascii1 = static_cast<int>(c1);
        int ascii2 = static_cast<int>(c2);
        int ascii = ascii1-ascii2;
        if(ascii<0) ascii +=126;
        if(ascii < 32 || ascii == 127)
            ascii = 48;                     //exception: carattere non stampabile
        char c = static_cast<char>(ascii);
        r+=c;
    }
    return r;
}