(protected) r = this^2, r != this (HAC 14.16)
function bnpSquareTo(r) {
  var x = this.abs();
  var x_array = x.array;
  var r_array = r.array;

  var i = r.t = 2*x.t;
  while(--i >= 0) r_array[i] = 0;
  for(i = 0; i < x.t-1; ++i) {
    var c = x.am(i,x_array[i],r,2*i,0,1);
    if((r_array[i+x.t]+=x.am(i+1,2*x_array[i],r,2*i+1,c,x.t-i-1)) >= BI_DV) {
      r_array[i+x.t] -= BI_DV;
      r_array[i+x.t+1] = 1;
    }
  }
  if(r.t > 0) r_array[r.t-1] += x.am(i,x_array[i],r,2*i,0,1);
  r.s = 0;
  r.clamp();
}