function bnpInvDigit() {
  if (this.t < 1) return 0
  var x = this[0]
  if ((x & 1) == 0) return 0
  var y = x & 3; // y == 1/x mod 2^2
  y = (y * (2 - (x & 0xf) * y)) & 0xf; // y == 1/x mod 2^4
  y = (y * (2 - (x & 0xff) * y)) & 0xff; // y == 1/x mod 2^8
  y = (y * (2 - (((x & 0xffff) * y) & 0xffff))) & 0xffff; // y == 1/x mod 2^16
  // last step - calculate inverse mod DV directly
  // assumes 16 < DB <= 32 and assumes ability to handle 48-bit ints
  y = (y * (2 - x * y % this.DV)) % this.DV; // y == 1/x mod 2^dbits
  // we really want the negative inverse, and -DV < y < DV
  return (y > 0) ? this.DV - y : -y
}