void signedAdd(SignedMutableBigInteger addend) {
        if (sign == addend.sign)
            add(addend);
        else
            sign = sign * subtract(addend);

    }
void signedAdd ( SignedMutableBigInteger addend ) { if ( sign == addend . sign ) add ( addend ) ; else sign = sign * subtract ( addend ) ; }
Signed addition built upon unsigned add and subtract.