code: private static final void mont_dbl(long10 t1, long10 t2, long10 t3, long10 t4,long10 bx, long10 bz) {
  		sqr(t1, t3);
  		sqr(t2, t4);
  		mul(bx, t1, t2);
  		sub(t2, t1, t2);
  		mul_small(bz, t2, 121665);
  		add(t1, t1, bz);
  		mul(bz, t1, t2);
  	}
docs: /* B = 2 * Q   where
  X(B) = bx/bz
  X(Q) = (t3+t4)/(t3-t4)
  clobbers t1 and t2, preserves t3 and t4