-1+2=3 (see below)

==== Here is the minimized test demonstrating the bug ====
import java.lang.Bignum;
class java_lang_Bignum_add {
  public static void main(String args[]) {
    Bignum b1=new Bignum("-1");
    Bignum b2=new Bignum("2");
    System.out.println(b1.add(b2)); // Should print "1"
  }
}
==== Output of the test ====
3