src/bigi/lib/bigi.js
function am1(i, x, w, j, c, n) {
  while (--n >= 0) {
    var v = x * this[i++] + w[j] + c
    c = Math.floor(v / 0x4000000)
    w[j++] = v & 0x3ffffff
  }
  return c
}
function am1(i, x, w, j, c, n) {
  while (--n >= 0) {
    var v = x * this[i++] + w[j] + c
    c = Math.floor(v / 0x4000000)
    w[j++] = v & 0x3ffffff
  }
  return c
}
am1: use a single mult and divide to get the high bits, max digit bits should be 26 because max internal value = 2*dvalue^2-2*dvalue (< 2^53)