(protected) set from integer value x, -DV <= x < DV
function bnpFromInt(x) {
  var this_array = this.array;
  this.t = 1;
  this.s = (x<0)?-1:0;
  if(x > 0) this_array[0] = x;
  else if(x < -1) this_array[0] = x+DV;
  else this.t = 0;
}